GIFT   OF 


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ELECTRICAL  MACHINERY 


BOOKS  BY 

TERRELL  CROFT 

PUBLISHED  BY 

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ELECTRICAL 

MACHINERY 


PRINCIPLES,  OPERATION  AND 
MANAGEMENT 


BY 
TERRELL  CROFT 

CONSULTING   ELECTRICAL   ENGINEER 

AUTHOR   OF   AMERICAN   ELECTRICIANS*  HANDBOOK,  WIRING  OF 
FINISHED    BUILDINGS,   WIRING  FOB  LIGHT  AND    POWER,   ETC., 


FIRST  EDITION 
FOURTH  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC. 

239  WEST  39TH  STREET.     NEW  YORK 


LONDON:   HILL  PUBLISHING  CO.,  LTD. 
6  &  8  BOUVERIE  ST.,  E.  C. 

1917 


COPYRIGHT,  1917,  BY  THE 
McGRAW-HiLL  BOOK  COMPANY,  INC. 


/\* 

\ 


THE  MAPLE  PRESS  YORK  PA 


PREFACE 

It  has  been  the  author's  experience  that  there  are  certain 
things  which  it  is  necessary  and  desirable  for  the  average  man 
to  know  about  electrical  machinery — and  it  is  entirely  possible 
to  transmit  this  essential  information  without  the  use  of  diffi- 
cult mathematics.  Furthermore,  there  are  a  great  many 
more  things  about  electrical  machines  which  (though  they  may 
be  interesting)  it  is  not  necessary  that  the  average  man  should 
know. 

In  preparing  this  " Electrical  Machinery"  book  we  have  en- 
deavored to  include  the  essential  and  desirable  things  and  to 
omit  the  non-essentials.  We  have  tried  to  explain  the  theo- 
retical principles  and  outline  the  operating  facts,  relating  to 
alternating-current  and  direct-current  generators  and  motors, 
and  similar  electrical  machines,  on  this  basis.  Control 
apparatus  has  been  given  due  attention.  Furthermore,  we 
believe  that  any  individual  who  can  read  English  will  be  able 
to  get  the  meat  from  this  volume  without  the  expenditure  of 
excessive  effort. 

Following  out  the  general  idea  above  disclosed,  much  of 
the  material  has  to  do  with  installation  and  operation — trouble 
location,  its  correction  and  the  like.  There  is  practically 
nothing  in  here  on  design  because  of  two  splendid  reasons: 
(1)  The  manufacturers  in  this  United  States  design,  build  and 
sell  perfectly  good  electrical  apparatus  much  more  effectively 
and  economically  than  can  any  one  who  is  not  regularly  en- 
gaged in  the  business.  Hence,  when  the  average  man  wants 
electrical  equipment  he  buys  it  on  the  market.  (2)  It  requires 
a  lot  of  special  knowledge  and  mathematics  to  design  electrical 
machinery. 

Summarizing:  ''Electrical  Machinery"  is  a  manual  of  ex- 
planation of  basic  theoretical  principles,  operation  and 

v 


388553 


vi  PREFACE 

management  prepared  particularly  to  assist  the  average  man 
who  is  now,  or  who  expects  sometime  to  be,  engaged  in 
practical  electrical  work. 

We  want  to  make  this  a  better  and  more  helpful  book  each 
time  it  is  revised  in  the  future.  Therefore,  if  any  reader  finds 
in  it  anything  which  he  cannot  understand,  will  he  please  write 
the  author  about  it?  Also,  if  you  find  any  errors — some  errors 
always,  regardless  of  how  carefully  the  checking  has  been  done, 
creep  into  every  technical  book — please  advise  us ;  they  will  be 
corrected  in  the  next  edition.  Finally,  we  will  be  most  grate- 
ful for  all  suggestions  for  the  future  enlargement  or  improve- 
ment of  the  book,  in  any  manner  whatsoever. 

TERRELL  CROFT. 

33  AMHERST  AVENUE,  UNIVERSITY  CITY, 
SAINT  Louis,  MISSOURI, 
June,   1917. 


ACKNOWLEDGMENTS 

The  author  desires  to  acknowledge  the  assistance  which  has 
been  rendered  by  a  number  of  concerns  and  individuals  in  the 
preparation  of  this  book. 

Considerable  of  the  material  appeared  originally  as  articles 
in  certain  trade  and  technical  periodicals  among  which  are: 
Practical  Engineer,  The  National  Electrical  Contractor ,  Power, 
Southern  Engineer,  Electrical  Review  and  Western  Electrician, 
Railway  Electrical  Engineer,  Electrical  Age  and  The  Power 
Plant.  Credit  for  this  prior  publication  is  hereby  accorded. 

Among  the  concerns  which  cooperated  in  supplying  text 
data  and  material  for  illustrations  are:  The  Ridgway  Dynamo 
&  Engine  Company,  The  Lincoln  Electric  Company,  The  West- 
inghouse  Electric  &  Manufacturing  Company,  The  Triumph 
Electric  Company,  The  Electric  Machinery  Company,  The 
General  Electric  Company,  The  Reliance  Electric  &  Engineer- 
ing Company,  The  Gurney  Ball  Bearing  Company.  Special 
acknowledgment  is  accorded  to  the  Wagner  Electric  & 
Manufacturing  Company  for  permission  to  incorporate  in  this 
book  the  substance  of  the  material,  relating  to  generator  and 
motor  testing,  from  its  publication,  A  Manual  of  Electrical 
Testing. 

Mr.  H.  Weichsel,  Chief  Designer,  Wagner  Electric  and 
Manufacturing  Company,  collaborated  with  the  author  in  the 
preparation  of  the  information  about  single-phase  motors. 
This  material  originally  appeared  as  two  articles  in  Electrical 
Review  and  Western  Electrician  for  May  12th  and  19th,  1917. 
Mr.  A.  C.  Lanier,  Chairman,  Department  of  Electrical  En- 
gineering, University  of  Missouri,  Columbia,  Mo.,  furnished 
some  data  concerning  the  compensated  direct-current  genera- 
tor. S.  C.  Wagner,  Superintendent  of  Distribution,  Electric 

vii 


viii  ACKNOWLEDGMENTS 

Company  of  Missouri,  read  the  galley  and  page  proofs,  called 
attention  to  a  number  of  errors,  and  suggested  numerous 
improvements. 

Other  acknowledgments  have  been  made  throughout  the 
book.  If  any  has  been  omitted  it  has  been  through  oversight 
and  if  brought  to  the  author's  attention  it  will  be  incorporated 
in  the  next  edition. 


CONTENTS 

PAGE 
PREFACE  v 

SECTION  1 

Principles,  Construction  and  Characteristics  of  Direct-Current  Gen- 
erators and  Motors 1 

SECTION  2 

Management  of  Direct-Current  Generators 50 

SECTION  3 

Management  of  and  Starting  and  Controlling  Devices  for  Direct- 
Current  Motors 70 

SECTION  4 
Troubles  of  Direct-Current  Generators  and  Motors 105 

SECTION  5 
Testing  of  Direct-Current  Generators  and  Motors 148 

SECTION  6 

Principles,  Construction  and  Characteristics  of  Alternating-Current 

Generators 157 

SECTION  7 

Management  of  Alternating-Current  Generators 178 

\ 

4 

SECTION  8 

Principles,  Construction  and  Characteristics  of  Induction  and  Re- 
pulsion Motors    .    .    . 190 

ix 


x  CONTENTS 

SUCTION  9 

PAGE 

Synchronous  Motors  and  Condensers 229 

SECTION  10 

Management  of,  and  Starting  and  Controlling  Devices  for  Alternating- 
Current  Motors 239 

SECTION.  11 

Troubles  of  Alternating-Current  Generators  and  Motors 263 

SECTION  12 
Testing  of  Alternating-Current  Generators  and  Motors 276 

SECTION  13 
Test  Determination  of  Motor-Drive  Power  Requirements 288 

SECTION  14 

Motor  Generators  and  Frequency  Changes 301 

INDEX    .  .   305 


ELECTRICAL  MACHINERY 

SECTION  1 

PRINCIPLES,  CONSTRUCTION  AND  CHARACTERISTICS 

OF  DIRECT-CURRENT  GENERATORS  AND 

MOTORS 

1.  An  Electrical  Generator  or  Dynamo*  is  a  machine  for 
converting  mechanical  power  into  electrical  power.     A  gen- 
erator develops  an  e.m.f.  by  cutting  lines  of  force  and  this 
e.m.f.  forces  a  current  to  flow  provided  the  external  circuit 
is  closed. 

2.  Performance  Specifications  are  furnished  by  the  manu- 
facturers of  generators  to  prospective  customers  which  indicate 
the  efficiencies,  temperature  rise,  etc.,  of  any  generator  that 
may  be  under  consideration.     These  specifications  show  what 
may  be  expected  of  the  machine  in  service  and  should  be 
thoroughly  studied  by  the  buyer. 

3.  Voltage  Regulation  (do  not  confuse  with  speed  regula- 
tion) is  the  ratio  of  the  change  of  voltage,  between  "no  load" 
and  "full  load,"   to  the  full-load  voltage.    It  is  usually  ex- 
pressed  as   a   percentage.     Thus,   the  speed  of  the  machine 
remaining  constant: 

,.,.    T,  ,  no-load  voltage  —  full-load  voltage 

(1)   Voltage  regu^on  =  -  fuii_Uad  voltage 

By  regulation  is  always  meant  some  change  which  a  machine 
makes  of  its  own  accord  when  the  load  is  changed.  This 
change  is  inherent  in  the  machine  and  is  determined  by  its 
construction. 

4.  Control  always  means  some  change  which  an  attendant 
brings  about  in  a  machine,  as,  for  instance,  the  raising  of  the 
voltage  by  cutting  resistance  out  of  the  field  circuit. 

*  A.  I.  E.  E.  STANDARDIZATION  RULE  No.  101,  June  28,  1916. 

1 


ELECTRICAL  MACHINERY 


[ART.  5 


5.  Direct-current  Generators  develop  a  direct  or  continuous 
e.m.f.,  that  is,  one  that  is  always  in  the  same  direction.     Com- 
mercial direct-current  generators  have  commutators  and  may 
thereby   be   distinguished   from   modern   alternating-current 
machines.     Additional  information  in  regard  to  commutation 
as  applied  to  direct-current  motors,  which  is  in  general  true  for 
direct-current  generators,  is  given  hereinafter. 

6.  Excitation  of  Generator  Fields. — To  generate  an  e.m.f., 
conductors  must  cut  a  magnetic  field  which  in  commercial 
machines  must  be  relatively  strong.     A  permanent  magnet 
can  be  used  for  producing  such  a  field  in  a  generator  of  small 
output,  such  as  a  telephone  magneto  or  a  generator  for  spark- 
ing for  an  automobile;  but  for  generators  for  light  and  power 
the  field  is  produced  by  electromagnets,  which  may  be  excited 
by  the  machine  itself  or  "separately  excited"  from  another 
source. 

7.  Series-wound    or    Constant-current    Generators   have 
their  armature  coils,  field  coils  and  external  circuits  in  series 


FJetd  Magnet 
Series-Field  Winding,, 


Series  Wound 
Generator 


FIG.  1. — Series-wound  generator. 

with  one  another.  See  Fig.  1.  Series  generators  are  now  used 
commercially  only  for  series  arc-lighting  circuits  and  are 
equipped  with  automatic  regulators  (Fig.  2)  to  maintain  the 
current  constant,  irrespective  of  the  resistance  of  the  external 
circuit,  or  the  number  of  lamps  in  service.  The  same  current 


SEC.  1] 


GENERATORS  AND  MOTORS 


passes  through  each  lamp  in  the  series  and  through  the  gen- 
erator. The  voltage  at  the  brushes  of  a  series  machine  is 
equal  to  (neglecting  a  small 

line   loss)    the   voltage    per         LSg£/  ras^K***     "~xw«!2! 
lamp  times  the  number  of 

Armffn/re 

lamps.  Thus  on  a  circuit 
of  100  lamps  each  requiring 
50  volts  the  brush  pressure 
would  be  100  X  50=  5,000 
volts.  As  shown  by  the  FIG.  2. — Essentials  of  brush-shift- 
r  f  TT  o  TT  x  mg  mechanism  for  a  constant-current 

graph   of   Fig.   3,  II,  up  to  generator. 

a   certain    maximum   value 

with  an  increase  in  load — resistance  in  this  case — the  voltage 

of  the  generator  increases,  tending  to  maintain  the  current 

320 


50      100     170     140     160     ISO     200 
Amperes  Load 
I-No  Armature  Resistance  or  Armature  Reaction 


0         12345 
Ohms  External  Resistance 
i-Showing  Critical  Resistance 


FIG.  3.  —  Characteristic  graphs  or  curves  of  a  series  generator. 

(The  graph  No.  I  of  I  indicates  the  relation  between  voltage  and  cur- 
rent if  there  is  no  armature  resistance  or  armature  reaction.  Hence, 
this  is  actually  a  no-load  saturation  graph  of  the  machine.  It  is  deter- 
mined by  separately  exciting  the  field  coils  so  that  no  current  flows  in 
the  armature.  Graph  No.  2  shows  the  actual  relation  between  terminal 
voltage  and  load  current.  The  total  voltage  drop  consists  of  that  por- 
tion due  to  the  decrease  in  flux  caused  by  armature  reaction  and  that 
required  to  send  the  current  through  the  armature,  brushes  and  series 
coils. 

In  the  graph  of  II  the  values  of  current  in  a  series  generator  and  the 
resistance  of  the  external  circuit  are  plotted.  The  critical  resistance 
of  this  particular  machine  is  4.9  ohms.  With  an  external  resistance 
greater  than  4.9  ohms,  the  machine  will  not  excite  itself  or  "build  up".) 

constant.    Automatic  regulation  to  maintain  constant  current 
is  usually  effected,  commercially,  by  either  shifting  the  brushes 


ELECTRICAL  MACHINERY 


[ART.  8 


or  by  cutting  in  and  out  portions  of  the  field  winding  or  by  a 
combination  of  the  two  methods. 

NOTE. — A  graph  is  a  line  representing  graphically  the  relation 
between  two  quantities  which  vary  simultaneously.  Graphs  were 
formerly  called  curves. 


Arc 


-Armature 


FIG    4. — Regulation  of  an  arc-lamp  machine  by  field  variation. 

8.  The  Essentials  of  an  Arrangement  for  Regulation  by 
Brush  Shifting  are  shown  in  Fig.  2.  (By  regulation  in  this 
case  is  meant  the  maintenance  of  a  constant  current.)  The 
course  of  the  main  current  is  indicated  by  the  heavy  line. 


,-Field  Frame 


FIG.  5. — Separately-excited  generator. 

When  the  current  is  at  normal  value  the  contactor  is  held  mid- 
way between  the  contacts  Ci  and  C2  by  the  spring.  If  the 
current  increases  slightly,  the  core  is  pulled  down  into  the 
solenoid  and  brings  the  contactor  with  it,  which  makes  contact 
with  C2.  This  permits  a  small  current  in  shunt  with  the 


SEC.  1] 


GENERATORS  AND  MOTORS 


solenoid  to  flow  through  the  clutch  B,  the  mechanical  details 
of  which  are  not  shown.  This  clutch  pulls  the  shifting  rod 
down  and  so  shifts  the  brushes  as  to  tend  to  maintain  the 
current  at  a  constant  value.  A  decrease  in  current  allows  the 


Separately— Exdteol 
Field  Terminals 


Brush  Holder. 


Frame-' 


r-MainLeaol 


FIG.  6.  —  A  separately-excited  generator  with  a  double  commutator 
for  electrolytic  work.  (General  Electric  Co.,  5  kw.,  6  or  12  volts,  835 
or  415  amp.,  1150  r.p.m.) 

spring  to  pull  the  contactor  against  d;  clutch  A  operates  and 
the  brushes  are  shifted  in  the  opposite  direction. 

9.  The  Principle  of  an  Arc  -light  (Constant-current)  Machine 
That  is  Regulated  by  Field  Variation  is  illustrated  in  Fig.  4. 
The   lever  L   is   shifted    auto- 

matically and  cuts  in  or  out 
turns  of  the  field  magnet  so  as 
to  maintain  a  constant  current 
in  the  external  circuit. 

10.  Separately   Excited 
Generators  are  used  for  electro- 
plating and  for  other  electro- 
lytic work  where  it  is  essential 
that  the  polarity  of  a  machine 
be   not  reversed.     Self-excited 
machines    may    change    their 
polarities.     The  essential  dia- 

grams are  shown  in  Fig.  5.  The  fields  may  be  excited  from 
any  direct-current,  constant-potential  source,  such  as  a  storage 
battery  or  lighting  circuit. 


'Armature 


•Ma in  Lead 


FIG. 


.h 

7. — Showing    connections 


ELECTRICAL  MACHINERY 


[ART.  11 


The  field  magnets  can  be  wound  for  any  voltage  because 
they  have  no  electrical  connection  with  the  armature.  With 
a  constant  field  excitation,  the  voltage  will  drop  slightly  from 
no-load  to  full-load  because  of  armature  drop  and  armature 


(Ventilating  Ducts 


"Shaft 


'^--Armature  Winding 


FIG.  8. — A  double-commutator  armature  for  a  low-voltage  "  electrolytic 

generator. 

reaction.  Inasmuch  as  the  machines  of  this  type  (Fig.  6)  used 
in  electrolytic  and  electroplating  work  may  carry  exceedingly 
large  currents,  they  are  frequently  provided  with  two  com- 
mutators as  shown  in  Figs.  7  and  8  so  that  the  current  may 


FIG.  9. — Shunt-wound  generator. 

be  carried  jointly  by  the  two  commutators.  That  is,  each 
commutator  should  carry  one-half  of  the  total  current  output 
of  the  machine. 

11.  The  Shunt-wound  Generator  is  shown  diagrammatic- 
ally  in  Fig.  9.     Shunt  generators  are  now  seldom  used.     They 


SEC.  1] 


GENERATORS  AND  MOTORS 


have  been  largely  superseded  by  compound-wound  machines. 
The  exciting  current,  a  small  part  of  the  total  current,  is 
shunted  through  the  fields.  The  exciting  current  varies  from 
possibly  5  per  cent,  of  the  total  current  in  small  machines  to 
1  per  cent,  in  large  ones.  The  exciting  current  is  determined 
by  the  voltage  at  the  brushes  and  the  resistance  of  the  field 
winding.  Residual  magnetism  in  the  field  cores  permits  a 
shunt  generator  to  ''build  up."  This  small  amount  of  magne- 
tism that  is  retained  in  the  field  cores  induces  a  voltage  in 
the  armature.*  This  voltage  sends  a  slight  current  through 
the  field  coils  which  increases  the  magnetization.  Thus,  the 
induced  voltage  in  the  armature  is  increased.  This  in  turn 
increases  the  current  in  the  fields,  which  still  further  increases 


320 
280|— ^ 

t» 

s  ?oo 

JI60 

hzo 


280 


900 


»600_  160 


300£  80 
40 


m 


300       600       900 

Amperes  Load 
I-  Shunt  Characteristic 


0     a?        0.6         1.0         14 

0.4        0.8         1.2         L6        2.0 
6  Ohms.  External  Resistance 
II-Effect  of  a  Short  Circuit 


FIG.  10. — Showing  characteristic  graphs  for  a  shunt-wound  generator. 

the  magnetization,  and  so  on,  until  the  saturation  point  and 
normal  voltage  of  the  machine  are  reached.  This  "building 
up"  action  is  the  same  for  any  self-excited  generator  and 
often  requires  20  to  30  seconds. 

12.  When  a  Shunt  Generator  Runs  at  Constant  Speed, 
as  more  and  more  current  is  drawn  from  the  generator,  the 
voltage  across  the  brushes  falls  slightly.  This  fall  is  due  to 
the  fact  that  it  requires  more  and  more  of  the  generated 
voltage  to  force  this  increasing  current  through  the  windings 
of  the  armature.  That  is,  the  armature  IR  drop  increases. 
This  leaves  a  smaller  part  of  the  total  e.m.f.  for  brush  e.m.f. 
Then  when  the  brush  pressure  falls'  there  is  a  slight  decrease 
in  the  field  current,  which  is  determined  by  the  brush  pressure. 
This  causes  the  total  e.m.f.  to  drop  a  little,  which  still  further 

*  ELEMENTS  OF  ELECTRICITY,  W.  H.  Timbie. 


8 


ELECTRICAL  MACHINERY 


[ART.  13 


lowers  the  brush  potential.  These  two  causes  combine  to 
gradually  lower  the  brush  pressure  (voltage)  especially  at  heavy 
overloads.  The  curve  in  Fig.  10  shows  these  characteristics. 
For  small  loads  the  curve  is  nearly  horizontal,  but  at  heavy 
overloads  it  shows  a  decided  drop.  The  point  where  the  out- 
put of  a  commercial  machine  drops  off  is  beyond  the  oper- 
ating range  and  is  only  of  theoretical  interest. 

13.  The  Voltage  of  a  Shunt  Machine  May  be  Kept  Fairly 
Constant  by  providing  extra  resistance,  R,  in  the  field  circuit, 
see  Fig.  11,  which  may  be  cut  out  as  the  brush  potential 
falls.  This  will  allow  more  current  to  flow  through  the  field 
coils  and  increase  the  number  of  magnetic  lines  set  up  in  the 
magnetic  circuit.  If  the  speed  is  maintained  constant,  the 
armature  conductors  cut  through  the  stronger  magnetic  field 

at  the  same  speed,  and  thus  in- 
duce a  greater  e.m.f .  and  restore 
the  brush  potential  to  its  for- 
mer value.  This  resistance 
may  be  cut  out  either  auto- 
matically or  by  hand. 

14.  A  Shunt-wound  Gener- 

FIG.  11. — Elementary  circuit  ator  Normally  Gives  a  Fairly 
of  a  shunt-wound  generator  and  ^  Tr  i 

load.  Constant    Voltage    even    with 

varying  loads,  and  can  be  used 

for  incandescent  lighting  and  other  constant-potential  loads. 
These  generators  do  not  operate  well  in  parallel,  partially 
because  the  voltage  of  one  machine  may  rise  above  that  of 
the  others  and  it  will  run  them  as  motors.  Shunt  generators 
running  in  parallel  do  not  " divide  the  load"  satisfactorily 
between  themselves.  They  are  seldom  installed  now,  as  com- 
pound-wound generators  are  more  satisfactory  for  most  pur- 
poses. Shunt  generators  may  be  bipolar  (two  poles)  or 
multipolar  (more  than  two  poles)  as  may  compound-wound 
generators.  See  the  following  Arts. 

15.  The  Compound-wound  Generator  is  shown  diagram- 
matically  in  Fig.  12.  If  a  series  winding  be  added  to  a  shunt 
generator  (Fig.  9)  the  two  windings  will  tend  to  maintain  a 
constant  voltage  as  the  load  increases.  The  magnetization 


SEC.  1] 


GENERATORS  AND  MOTORS 


9 


due  to  the  series  windings  increases  as  the  line  current  increases, 
which  will  cause  the  voltage  generated  by  the  armature  to 
rise.  The  drop  of  voltage  at  the  brushes  that  occurs  in  a 


-Field  Frame 


-  *•  Shunt-Field  vnhaing 
Compound  Wound 
Generator 
Series-Field  Winding 


Shunt  F:elct.       Rheostat 


Line 
5witch 


FIG.  12. — Compound-wound  generator. 

shunt  generator  is  thus  compensated  for.     See  also  Fig.  13 
and  Art.  76. 

16.  A  Flat-compounded  Compound -wound  Generator  is  one 
having  its  series  coils  so  proportioned  that  the  voltage  (Fig. 
15)    remains    practically    con- 
stant  at   all   loads  within   its 

range. 

17.  An   O v e r -compounded 
Generator  has  its  series  wind- 
ings so  proportioned  that  its 
full-load   voltage    (Fig.    14)   is 
greater  than  its  no-load  volt- 
age.    O  ver-c  ompoundingis 
necessary  where  it  is  desirable 

to  maintain  a  practically -con-  yIG<   13.— Elementary  connec- 

stant    voltage    at    some    point  tions    for   parallel   operation   of 

,,       v          ,.  ,       ,    -  compound-wound  generator, 
out  on  the  line  distant  from 

the  generator.     It  compensates  for  line  drop. 

18.  The    Characteristic    Curve    of    a    Compound -wound 
Machine  (Fig.  15)  indicates  how  the  terminal  voltage  is  due 
to  the  action  of  both  shunt  and  series  windings.     The  voltage 


.,, 
Field 

''Series  Shunt 
'EqualizerSwifth 


10 


ELECTRICAL  MACHINERY 


[ART.  18 


of  the  compound  generator  at  any  load  (AD)  is  equal  to  the 
sum  of  the  voltage  due  to  shunt  winding  (AC)  plus  that  due 
to  the  series  winding  (AB).  Generators  are  usually  over-com- 
pounded so  that  the  full-load  voltage  is  from  5  to  10  per  cent, 
greater  than  the  no-load  voltage.  See  Fig.  14. 


2J5 


215 


No  Load 


jLoaa/ 


Load 


fLoaet 


FullLoact- 


FIG.  14. — Voltage  graphs  of  an  "  over-compounded  generator.     (In  prac- 
tice most  compound-wound  generators  are  thus  over-compounded.) 

Although  compound-wound  generators  are  usually  provided 
with  a  field  rheostat,  it  is  not  intended  for  regulating  voltage 
as  the  rheostat  of  a  shunt-wound  machine  is.  It  is  provided 
to  permit  of  initial  adjustment  of  voltage  and  to  compensate 
for  changes  of  the  resistance  of  the  shunt  winding  caused  by 


40 60 80  100          J20 

Amperes 

FIG.  15. — Characteristic    graph    for    a    compound-wound    generator. 
(This  graph  is  from  a  flat  compound  machine.) 

heating.  With  a  compound-wound  generator,  the  voltage 
having  been  once  adjusted,  the  series  coils  automatically 
strengthen  the  magnetic  field  as  the  load  increases.  For  di- 
rect-current power  and  lighting  work,  compound-wound  gen- 
erators are  used  almost  universally. 


SEC.  1]  GENERATORS  AND  MOTORS  11 

19.  If  a  Compound-wound   Generator  is   Short-circuited 

the  field  strength  due  to  the  series  windings  will  be  greatly 
increased,  but  the  field  due  to  the  shunt  winding  will  lose  its 
strength.  For  the  instant  or  so,  that  the  shunt  magnetization 
is  diminishing,  a  heavy  current  will  flow.  If  the  shunt  mag- 
netization is  a  considerable  proportion  of  the  total  magneti- 
zation, the  current  will  decrease  after  the  heavy  rush  and 
little  harm  will  be  done  if  the  armature  has  successfully  with- 
stood the  heavy  rush.  However,  if  the  series  magnetization 
is  quite  strong  in  proportion  to  the  shunt,  their  combined 
effect  may  so  magnetize  the  fields  that  the  armature  will  be 
burnt  out. 


Series  field 


t -Field  Winding 

FIG.  16. — "Short-shunt"  method  FIG.  17. — Long-shunt  com- 

of  connecting  a   compound-wound  pound-wound  generator, 

generator. 

20.  A  Short-shunt  Compound-wound  Generator  (Fig.  16) 
has  its  shunt  field,  F,  connected  directly  across  the  brushes, 
A  and  B.     Generators  are  usually  connected  in  this  way  be- 
cause it  tends  to  maintain  the  shunt-field  current  more  nearly 
constant  on  variable  loads,  as  the  drop  in  the  series  winding 
does  not  directly  affect  the  voltage  impressed  on  the  shunt  field. 

21.  A  Long-shunt   Generator  has  its  shunt-field  winding 
connected  across  the  terminals  of  the  generator.     See  Fig.  17. 

22.  The  Compounding  of  a  Direct-current  Generator  Will 
Change  with  Its  Speed*  because  the  location  of  the  no-load, 
normal- voltage  point  (A,  Fig.  18)  on  the  magnetization  graph 
is  determined  by  the  speed.     Thus  consider  the  following 
example : 

*  Gordon  Fox  in  ELECTRICAL  REVIEW  AND  WESTERN  ELECTRICIAN. 


12 


ELECTRICAL  MACHINERY 


[ART.  23 


EXAMPLE. — Point  A  (Fig.  18)  represents  the  magnetization  corre- 
sponding to  an  e.m.f.  of  230  volts  without  load  at  250  r.p.m.  The 
series  ampere-turn  magnetization  is  represented  by  AB.  The  full-load 
voltage  is  higher  than  the  no-load  voltage  by  an  increment  or  increase 
BC,  which  represents  the  amount  of  overcompounding.  Assume  that 
the  generator  speed  is  increased  to  275  r.p.m.  The  no-load  e.m.f.  cor- 
responding to  230  volts  at  the  terminals  would  now  be  about  10  per  cent, 
lower  because  the  speed  has  been  increased  10  per  cent.  This  point  is 
at  D.  The  number  of  series  ampere-turns  at  full-load  is  the  same  shown 
as  before,  the  distance  DE  (equal  to  AB)  representing  this  magnetizing 
influence. 

The  over-compounding  is  here  shown  by  the  increment  EF,  It  will 
be  noted  that  the  distance  EF  is  considerably  greater  than  BC.  This 
means  that  the  generator  will  over-compound  to  a  greater  degree  when 


Field  Ampere-Turns 
FIG.  18. — Magnetization  graph 
of  a  220-volt  direct-current  gener- 
ator   normally   operating    at  250 
r.p.m. 


220 
' 


FIG.  19.  —  Graphs  for  the  same 
generator  showing  voltage  regu- 
lation at  different  speeds. 


operated  at  275  r.p.m.  than  when  driven  at  250  r.p.m.,  other  conditions 
being  unchanged.  The  distance  EF  does  not  represent  the  voltage  rise 
accurately.  It  represents  the  increase  in  magnetic  density  from  no-load 
to  full-load.  An  equal  increase  in  magnetic  density  will  cause  a  greater 
increase  in  voltage  at  275  r.p.m.  than  at  250  r.p.m.  Hence,  the  rise  in 
voltage  at  the  increased  speed  is,  in  reality,  greater  than  is  indicated  on 
the  graph. 

23.  The  Voltage  Regulation  for  the  Same  Generator 
Operating  at  Two  Different  Speeds*  is  shown  by  the  graphs 
of  Fig.  19.  The  curve,  X,  for  the  generator  operating  at  250 
r.p.m.  is  here  similar  to  AC  of  Fig.  18.  '  This  curve  is  not 
strictly  accurate,  as  engine  regulation,  armature  reaction  and 
internal  resistance  tend  to  cause  the  voltage  to  decrease  with 

*  Gordon  Fox  in  ELECTRICAL  REVIEW  AND  WESTERN  ELECTRICIAN. 


SEC.  1] 


GENERATORS  AND  MOTORS 


13 


load.  The  increase  in  terminal  voltage  due  to  over-com- 
pounding, renders  the  shunt-field  influence  greater  under  full- 
load  conditions.  This  latter  factor  about  offsets  those  first 
mentioned,  leaving  the  curve,  as  shown,  nearly  unchanged. 
The  curve,  Y,  showing  the  voltage  regulation  of  the  generator 
when  driven  at  275  r.p.m.,  is  obtained  from  the  portion  DF 
in  Fig.  18.  The  values  have  been  modified,  however,  by  in- 
creasing the  magnetic  density  increments,  such  as  EF,  by  the 
ratio  275 :  250  to  correct  for  increase  in  speed. 

24.  The  Portion  of  the  Magnetization  Curve  Over  Which 
a  Generator  Operates  Determines  the  Contour  of  its  Voltage- 
regulation  Curve,*  and  this  may  materially  affect  parallel 


15%  400£ 

Per  Cent  Load 


125% 


1507. 


FIG.  20. — Graphs  showing  regulation  of  two  generators  both  compounded 
to  impress  the  same  voltage  at  full  load. 


operation.  In  Fig.  20  are  shown  two  voltage-regulation  curves 
of  two  generators  having  equal  voltage  rise  from  no-load  to 
full-load.  They  are  both  equally  over-compounded.  But,  at 
fractional  loads,  for  instance  at  half-load,  CD,  there  is  consid- 
erable difference  in  terminal  voltage.  If  these  two  generators 
were  connected  in  parallel,  machine  A  would  take  a  greater 
share  of  the  load  at  half-load.  To  secure  correct  load  division 
over  the  entire  operating  range,  two  or  more  generators  must 
have  compounding  curves  which  are  similar.  From  what  has 
preceded  it  is  evident  that  by  adjustments  of  prime-mover 
speed  it  is  possible  to  modify  the  compounding  curves  so  as 
to  improve  the  load  division. 

*  Gordon  Fox  in  ELECTRICAL  REVIEW  AND  WESTERN  ELECTRICIAN. 


14 


ELECTRICAL  MACHINERY 


[ART.  25 


25.  Speed  Changes  Have  an  Effect  on  the  Temperature  of 
a  Generator.*  When  operating  at  increased  speeds  the  iron 
is  worked  lightly,  hence  core  losses  are  low.  The  increase  in 


Armature  ~, 


/Pole  Piece 


I-  Cumulative  Compound 


H~Differential  Compound 


I  Circuit 
/L  •  Breaker 


G.D.Lamps 


FIG.  21. — Showing  cumulative  and  differential-compound  windings  for 
direct-current  generators  and  motors. 

windage  and  ventilation  also  tends  toward  cooler  operation. 
When  the  machine  is  operated  below  normal  speed,  it  will 
tend  to  run  warmer.  The  shunt-field  coils  in  particular  will 

be  affected,  since  more  shunt  am-, 
pere-turns  are  required  to  secure 
the  desired  no-load  voltage. 

26.  Differential  and  Cumulative - 
compound  Windings  for  direct-cur- 
rent motors  and  generators  are  illus- 
trated diagrammatically  in  Fig.  21. 
With  a  machine  which  is  wound  dif- 
ferentially the  shunt  and  series  fields 
oppose  each  other  (77),  and  as  the 
load  increases,  the  electromagnetic 
field  due  to  the  series  winding  would 
\  increase  correspondingly  and  "  over- 
pound-wound  generators  to  come"  the  field  due  to  the  shunt 
house)  *  "b  °  a  r  d  (Westing"  winding.  This  would  tend  to  change 

the  direction  of  the  magnetic  field 

in  the  air  gap  and  reverse  the  direction  of  the  e.m.f.  of  a 
generator  or  the  direction  of  rotation  of  a  motor.  Where 
machines  are  "cumulative-compound"  wound,  as  at  7,  the 

*  Gordon  Fox  in  ELECTRICAL  REVIEW  AND  WESTERN  ELECTRICIAN. 


SEC.  1] 


GENERATORS  AND  MOTORS 


15 


series  and  shunt-field  windings  " assist"  instead  of  "oppose" 
each  other. 

27.  A  Series  Shunt  for  a  Compound -wound  Generator  con- 
sists of  a  low-resistance  conductor  arranged  across  the  termi- 
nals of  the  series  field  (see  Figs.  13  and 

22)  by  means  of  which  the  compound- 
ing effect  of  the  series  winding  may  be 
regulated  by  shunting  more  or  less  of 
the  armature  current  past  the  series 
coils.  The  series  shunt  may  be  in  the 
form  of  grids,  on  large  machines,  or 
for  smaller  generators  in  the  form  of 
ribbon  resistors.  In  the  latter  case 
the  resistors  are  usually  insulated  and 
folded  into  small  compass. 

28.  Nearly  All   Commercial  Direct- 
current  Generators  and  Motors  Have 


Direction       of 
Magnetic    Flux. 

FIG.  23. — Magnetic 
circuits  of  a  four-pole 
generator. 


More  Than  Two  Poles. — In  some  of  the  preceding  diagrams 
only  two  were  shown  so  that  the  diagrams  would  be  simple. 
A  two-pole  machine  is  a  bipolar  machine;  one  having  more 
than  two  poles  is  a  multipolar  machine.  Fig.  23  shows  the 
direction  of  the  magnetic  flux  of  a  four-pole  machine.  Dia- 


t-— Frames--. 

1-  Old  Bipolar  Machine          E-Modern  Bipolar  Machine  E- Multipolar  Machine 

FIG.  24. — Direction  of  field  windings  on  generator  frames. 

grams  for  machines  having  more  poles  would  be  similar.  In 
multipolar  machines  there  is  usually  one  pair  of  brush  sets 
for  each  pair  of  poles,  but  with  series-wound  armatures,  such 
as  are  frequently  used  for  railway,  automobile  and  crane 
motors,  one  set  of  brushes  may  suffice  for  a  multipolar 


16  ELECTRICAL  MACHINERY  [ART.  29 

machine.  The  connections  of  different  makes  of  machines 
vary  in  detail  and  the  manufacturers  will  always  furnish 
complete  diagrams,  so  no  attempt  will  be  made  to  give  them 
here.  The  directions  of  the  field  windings  on  generator  frames 
are  given  in  Fig.  24.  The  directions  of  the  windings  on 
machines  having  more  than  four  poles  ^re  similar,  in  general, 
to  those  of  the  four-pole  machines? 

29.  Direct-current  Generators  May  be  Classified  Into: 
(1)  Non-commutating  Pole;  (2)  Commutating  Pole;  and  (3) 
Compensated,  as  regards  their  commutating  characteristics. 
In  non-commutating-pole  generators  no  special  provision  is 
made  to  insure  the  existence  of  a  flux  to  produce  sparkless 


Series    Interpo/e 

Winding    Winding     Arma+un 


Equalizer— 

Negative--'       '-Positive 

I  Wiring  on  Frame 

FIG.  25. — Diagram    of    compound-wound    commutating-pole  machines. 

commutation — except  that  this  condition  may  be  partially 
realized  by  shifting  the  brushes  of  the  machine.  With  com- 
mutating-pole generators  the  auxiliary  poles  produce  a  flux  at 
such  a  location  that  it  will — as  hereinafter  described — practir 
cally  eliminate  brush  sparking.  The  compensated  generator 
(Art.  37),  in  addition  to  having  commutating  poles,  has  con- 
ductors to  neutralize  the  effect  of  armature  reaction,  imbedded 
in  the  main  pole  faces. 

30.  Commutating-pole  Generators  and  Motors,  Fig.  25.* — 
The  principal  advantage  of  the  commutating-pole  construction 
resides  in  the  fact  that  with  it  the  commutation  can  be  ren- 
dered practically  perfect  under  any  condition  of  service. 

*  STANDARD  HANDBOOK. 


SEC.  1] 


GENERATORS  AND  MOTORS 


17 


Pole 


Pole 


31.  The  Object  in  Using  the  Commutating  Poles*  is  to 
produce  within  the  armature  coil  under  commutation  an  e.m.f. 
of  the  proper  value  and  sign  to  reverse  the  current  in  the  coil 
while  it  is  yet  under  the  brush — a  result  that  is  essential  to 
perfect  commutation.  The  variation  in  the  flux  distribution 
in  the  air  gap  of  a  commercial  direct-current  machine  of  the 
ordinary  shunt-wound  type,  at  no-load  and  under  full-load, 
is  shown  in  Fig.  26.  Consider  now  the  value  and  position 
of  the  flux  in  the  coil  under  the  brush  when  the  machine  is 
operating  at  full-load.  The  motion  of  the  armature  through 
this  flux  causes  the  generation  within  the  coil  of  an  e.m.f., 
and  the  sign  of  this  e.m.f.  is  such  as  to  tend  to  cause  the 
current  in  the  coil  to  continue 
in  the  direction  which  it  had 
before  the  coil  reached  the 
brush,  and  hence  it  opposes 
the  desired  reversal  of  the  cur- 
rent before  'the  coil  leaves  the 
brush. 

There  is  an  additional  detri- 
mental influence  which  tends  to 

retard  the  rapid  reversal  of  the  FIG.  26.— Distribution  of  mag- 
current  even  when  all  other  in-  netic  flu*  at  no  load  and  at  full 
n  ,  m  .  load,  without  commutatmg  poles, 

fluences    are    absent.     This 

latter  influence  is  due  to  the  local  magnetizing  effect  of  the 
current  in  the  coil  under  the  brush.  On  account  of  this,  lines 
of  force  surround  the  conductor,  the  change  in  the  intensity 
of  which  lines,  with  the  fluctuations  of  the  current  as  it  tends 
to  be  reversed,  generates  in  the  coil  an  e.m.f.  which  opposes 
the  change  in  the  intensity  of  the  current.  This  reactive 
e.m.f.  is  in  the  same  direction  as  that  due  to  the  cutting  of 
the  flux  by  the  coil  under  the  brush  and  is  likewise  propor- 
tional to  the  speed. 

It  will  be  apparent  that  even  were  the  field  distortion  com- 
pletely neutralized,  the  detrimental  reactive  e.m.f.  would  yet 
remain.  The  improved  and  practically  perfect  commutation 
of  a  commutating-pole  machine  is  due  to  the  fact  that  the 

•  WESTINGHOUSE  PUBLICATION. 

2 


90          180          270 
El  ectrkal  Space  Degrees. 


360 


18 


ELECTRICAL  MACHINERY 


[ART.  32 


Inter- 


flux,  which  is  locally  superposed  upon  the  main  field,  not  only 
counterbalances,  the  undesirable  main  flux  cut  by  the  coil 
under  the  brush,  but  it  causes  to  be  generated  within  the  coil 
an  e.m.f.  sufficient  to  equal  and  oppose  the  reactive  e.m.f.  just 
referred  to.  This  effect  will  be  appreciated  from  a  study  of 
Fig.  27,  which  represents  the  distorted  flux  of  the  motor  of  the 
usual  design,  as  shown  in  Fig.  26,  and  indicates  the  results  to 
be  expected  when  the  flux  due  to  the  auxiliary  or  commutatirig 
pole  is  given  the  relatively  proper  value. 

The  effect  of  the  commutating  poles  is  the  more  pronounced 
the  weaker  the  main  field;  and  the  commutation    voltage 

thereby  induced  if  correct  for 
a  low  speed,  is  correct  for  a 
high  speed.  With  increase  of 
load-current  and  main-field  dis- 
tortion there  is  a  proportional 
increase  of  counter-magnetiz- 
ing field  produced  in  the  coil 
under  the  brush,  up  to  the 
point  of  magnetic  saturation 
of  the  auxiliary  or  commutat- 
ing pole.  Sparkless  operation 
is  insured  for  all  operating 
ranges  both  of  speed  and  load. 
32.  Commutating-pole,  Direct-current  Generators  are  simi- 
lar in  construction  and  operation  to  commutating-pole  motors. 
Ordinary  generators*  that  operate  under  severe  overloads  and 
over  a  wide  speed  range  are  liable  to  spark  under  the  brushes 
at  the  extreme  overloads  and  at  higher  speeds.  This  is  because 
the  field  due  to  the  armature  current  distorts  the  main  field  to 
such  an  extent  that  the  coils  being  commutated  under  the 
brush  are  no  longer  in  a  magnetic  field  of  the  proper  direction 
and  strength.  To  overcome  this,  "interpoles"  (Figs.  28  and 
29)  are  placed  between  the  main  poles.  See  Fig.  25.  These 
commutating  poles  introduce  a  magnetic  field  of  such  direction 
and  strength  as  to  maintain  the  magnetic  field,  at  the  point 
where  the  coils  are  commutated,  at  the  proper  strength  for 

*  Westinghouse  Elec.  &  Manfg.  Co. 


90  180         270        360 

Electrical  Space  Degrees. 

FIG.  27. — Distribution  of  mag- 
netic flux  at  full  load,  with  and 
without  commutating  poles. 


SEC.  1] 


GENERATORS  AND  MOTORS 


19 


Frame. 


E- Bottom  View 

FIG.  28.— Westinghouse  method  of  constructing  main  poles  for  medium 
and  large  direct-current  generators. 


1  -Insulated  Bo/t  insulation  on 

I-Front  Elevation  &><*  Turns 

H-Side  Elevation 

Insulation  on< 
End  Turns 
H-Bottom  View 

».  29. — Details  of  construction  of  a  commutating  pole  on  a  Westing- 
house  generator. 


20 


ELECTRICAL  MACHINERY 


[ART.  33 


perfect  commutation.  Commutating  poles  are  sometimes 
called  "interpoles"  but  "commutating  poles"  is  the  preferable 
term. 

33.  The  Winding  of  the  Commutating  Poles  is  connected 
in  series  with  the  armature  so  that  the  strength  of  the  cor- 
rosive commutating-pole  field  is  proportional  to  the  load.     The 
adjustment  and  operation  of  commutating-pole  generators  is 
not  materially  different  from  that  of  non-commutating-pole 
machines. 

34.  When  the  Brush  Position  of  a  Commutating-pole  Ma- 
chine Has  Once  Been  Properly  Fixed,  No  Shifting  is  After- 
ward Required  or  should  be  made,  and  most  of  these  gen- 


Armature 


Direction  of 

Ma  in  Pole 

Flux 


FIG.  30. — Distribution  of  flux  in  a   commutating-pole  generator. 


erators  are  shipped  without  any  shifting  device.  An  arrange- 
ment for  securely  clamping  the  brush-holder  rings  to  the  field 
frame  is  provided. 

35.  In  Commutating-pole  Apparatus  Accurate  Initial  Ad- 
justment of  the  Brush  Position  is  Necessary. — The  correct 
brush  position  is  on  the  no-load  neutral  point,  which  is  located 
by  the  manufacturer.  A  templet  is  furnished  with  each  ma- 
chine, or  some  other  provision  is  made  whereby  the  correct 
brush  location  can  be  determined  by  the  installer.  If  the 
brushes  are  given  a  backward  lead  on  a  commutating-pole 
generator,  the  machine  will  over-compound  and  will  not  corn- 
mutate  properly.  With  a  forward  lead  of  the  brushes,  a 


SEC.  1] 


GENERATORS  AND  MOTORS 


21 


generator    will    under-compound    and    will    not   commutate 
properly. 


Armature- 
l-Commutoiting  Pole  Machine 

FIG.  31. — Showing  commutating  and  compensated  pole  direct-current 

generators, 

36.  The  Action  of  the  Magnetic  Flux  in  a  Commutating-pole 
Generator  is  illustrated  in  Fig.  30.     The  direction  of  the  main 
field  flux  is  shown  by  the  dashed  line.     The  direction  of  the 
armature     magnetization     is 

shown  by  the  dotted  lines. 
The  direction  of  the  flux  in  the 
interpole  is  shown  by  the  full 
line.  It  is  evident  that  the 
interpole  flux  is  in  a  direction 
opposite  to  that  of  the  arma- 
ture flux,  and  as  the  interpole 
coil  is  more  powerful  in  its 
magnetizing  action  than  the 
armature  coils,  the  flux  of  the 
armature  coils  is  neutralized. 
With  a  less  powerful  magne- 
tizing force  from  the  interpole 
than  from  the  armature,  the 
armature  would  overpower  the 
interpole  and  reverse  the  direc- 
tion of  the  flux,  which  would 
result  in  an  unsatisfactory 
commutating  condition. 

37.  The  Compensated  Generator*  has,  by  virtue  of  its  com- 
pensated winding  which  is  located  in  the  main  pole  faces 

•  See  article  "  The  Compensated  Generator"  by  David  Hall  in  PRACTICAL  ENGINEER 
for  Sept.  15,  1916. 


FIG.  32.— Portion  of  the  stator 
of  a  "compensated."  Direct-cur- 
rent generator  showing  magnetiz- 
ing conductors  embedded  in  the 
main-pole  faces. 


22  ELECTRICAL  MACHINERY  [ART.  38 

(Figs.  31  and  32),  the  property  within  itself  of  compensating 
armature  reaction.  That  is,  it  neutralizes  the  magnetizing 
effect  of  the  armature  winding.  In  the  compensated  machine, 
conductors  in  series  with  the  armature  winding  are  also  im- 
bedded in  the  main  pole  faces,  as  suggested  in  the  illustration. 
With  the  conductors  thus  located  in  the  pole  faces,  the  effect 
of  armature  magnetization  and  the  distortion  of  flux  incident 
thereto  may  be  more  effectively  "neutralized"  than  with 
commutating  poles  alone. 

NOTE. — It  should  be  noted*  that  the  commutating-pole  machine  and 
the  compensated  machine  are  distinct  forms.  The  compensating  winding 
and  the  commutating  poles  may  or  may  not  be  combined  in  the  same 
machine.  That  is,  a  "compensated,"  direct-current  motor  or  generator 
may  be  designed  without  commutating  poles.  It  is,  however,  the 
almost-universal  practice  in  designing  modern  direct-current  machines 
to  provide  a  commutating  pole  and  winding  or  to  provide  a  central 
tooth  with  heavy  excitation  to  generate  commutating  flux  over  the 
commutation  zone  regardless  of  whether  or  not  a  compensating  winding 
is  used. 

The  compensating  windings  are,  in  general,  particularly  desirable  only 
for  machines  in  which  the  voltage  between  commutator  bars  is,  for 
some  reason  or  other,  relatively  great.  Hence,  they  are  applied  to  high- 
voltage  machines  or  to  machines  which,  due  to  severe  operating  condi- 
tions, would  be  subjected  to  excessive  voltage  between  the  commutator 
bars  under  the  pole  tip.  Without  compensation,  the  coils  connecting  to 
such  bars  would,  at  the  instant  of  commutation,  be  cutting  a  magnetic 
field  of  high  density  because  of  armature  reaction.  Furthermore,  the 
increase  in  iron  losses,  in  the  armature  teeth,  at  full  load  over  their  no- 
load  losses,  which  results  from  flux  distortion  is,  in  non-commutating- 
pole  machines,  largely  eliminated  by  the  use  of  the  compensating 
winding.  This,  in  certain  cases,  may  render  it  possible  to  dispose  the 
active  material  in  the  armature  somewhat  more  economically  in  com- 
pensated machines  than  would  be  possible  in  machines  of  the  non- 
compensated  types. 

38.  Three -wire  Direct-current  Generators  f  are  ordinary 
direct-current  generators  (Figs.  33,  34  and  35)  with  the 
modifications  and  additions  (Fig.  36)  described  below. 
They  are  usually  wound  for  125-250- volt  three- wire  circuits. 
In  Westinghouse  three-wire  generators  four  equidistant  taps 

*  A.  C.  LANIEK. 

t  Westinghouse  Elec.  &  Manfg.  Co. 


SEC.  1] 


GENERATORS  AND  MOTORS 


23 


FIG.  33. — Diagram  of  a  three- 
wire  generator  showing  connec- 
tions for  shunt  and  series  coils 
and  balance  coil. 


FIG.  34. — Another  type  of 
three-wire  generator  operating 
independently. 


FIG.  35. — A  double-commutator,  single-armature,  three-wire   generator 
(125  volts  per  commutator). 


Outside  Wire- 
Incandescent-- 
Lamp  Loaol* 

Neutral  Wire^ 


?ZO-Volt  Motor-, 

-C7> 

•  '.6-  • 


O^side  Wire-' 


FIG.  36. — Diagram  illustrating  the  principle  of  the  three-wire  generator. 


24 


ELECTRICAL  MACHINERY 


[ART.  38 


Balance 


..^ 
inqs       field 

FIG.    37.— Diagram     showing  FIG.  38.— Showing  fundamental 

connections  for  three-wire  gen-          connections   for    a    Westinghouse 
erator.  three-wire  generator. 


-Eye  Bolt 


Cora 


Field  frame*' 


VT 

FIG.  39. — Sectional  elevation  of  Westinghouse  three-wire  generator. 


SEC.  1] 


GENERATORS  AND  MOTORS 


25 


are  made  in  the  armature  winding,  and  each  pair  of  taps 
diametrically  opposite  each  other  is  connected  together 
through  a  balance  coil.  See  Figs.  37  and  38.  The  middle 
points  of  the  two  balance  coils  are  connected  together  and 
this  junction  constitutes 
the  neutral  point  to 
which  the  third  or  neu- 
tral wire  of  the  system 
is  connected.  A  con- 
stant voltage  is  main- 
tained between  the  neu- 
tral and  outside  wires 
which,  within  narrow 
limits  (Fig.  36)  is  one- 
half  the  generator  volt-  FIG>  4a_One  Three_wire  direct-current 
age.  The  generator  generator,  125-250  volts  in  parallel  with  two 
shaft  is  extended  at  the  ^SS^SS^^  125  V°lts-  Diagram 
commutator  end  for  the 

collector  rings  (Fig.  39).  Four  collector  brushes  and  brush 
holders  are  used  in  addition  to  the  regular  direct-current 
brushes  and  brush  holders. 


Ground 
'Shunt  Ffdd 


F'ld 


f< 


Generating  Station 


••Line —  >K" •••Load 


*  Series  Fie  la/- 


FIG.  41. — Connections  of  a  three-wire  generator  operating  in  parallel 
with  a  two-wire  generator. 

39.  The  Series  Coils  of  Compound-wound  Three-wire 
Generators  Are  Divided  into  Halves  (see  Figs.  40  and  41), 
one  of  which  is  connected  to  the  positive  and  one  to  the 
negative  side.  This  is  done  to  obtain  compounding  on 


26 


ELECTRICAL  MACHINERY 


[ART.  40 


either  side  of  the  system  when  operating  on  an  unbalanced 
load.  To  understand  this,  consider  a  generator  with  the 
series  field  in  the  negative  side  only  and  with  most  of  the 
load  on  the  positive  side  of  the  system.  The  current  flows 
from  the  positive  brush  through  the  load  and  back  through 
the  neutral  wire  without  passing  through  the  series  field. 
The  generator  is  then  operating  as  an  ordinary  shunt  machine. 
If  most  of  the  load  be  on  the  negative  side,  the  current  flows 
out  the  neutral  wire  and  back  through  the  series  fields, 
boosting  the  voltage  on  that  side  only.  Such  operation  is 
evidently  not  satisfactory,  and  so  the  divided  series  fields 
are  provided. 

40.  Wires  Connecting  the  Balance  Coils  to  a  Three- 
wire  Generator  must  be  short  and  of  low  resistance.  Any 
considerable  resistance  in  these  will  affect  the  voltage  regula- 
tion. The  unbalanced  current  flows  along  these  connections; 

consequently,  if  they  have 
much  resistance,  the  re- 
sulting drop  in  voltage 
reduces  the  voltage  on  the 
heavily  loaded  side. 

41.  Switches  Are  Not 
Ordinarily  Placed  in  the 
Circuits  Connecting  the 
Collector  Rings  to  the 
Balance  Coils.  —  When 
necessary,  the  coils  may 
be  disconnected  from  the 
generator  by  raising  the 
brushes  from  the  collector 


FIG.  42. — One  three-wire  direct  cur- 
rent generator,  125-150  volts,  in  par- 
allel with  one  two-wire  generator,  250 
volts.  Diagram  of  connections. 


rings.  Switching  arrangements  often  make  it  necessary  to  run 
the  balance-coil  connections  to  the  switchboard  and  back. 
This  necessitates  heavy  leads  to  keep  the  drop  low.  If 
heavy  leads  are  not  used,  then  poor  regulation  may  result. 
The  balance  coils  are  so  constructed  that  there  is  very  little 
likelihood  of  anything  happening  to  them  that  will  not  be 
taken  care  of  by  the  main  circuit-breakers.  Complete 
switchboard  connection  diagrams  are  given  in  Figs.  40,  42  and 


SEC.  1] 


GENERATORS  AND  MOTORS 


27 


43.     Fig.  44  shows   a   simplified  diagram  of  two  three-wire 
generators  arranged  for  parallel  operation. 

42.  Commutating-pole,  Three-wire  Generator  Connections 
(Fig.  45)  are  so  made  that  one-half  of  the  commutating- 
pole  winding  is  in  the  positive  side  and  the  other  half  is  in 


Am. 
Shunfs 


FIG.  43. — Diagram  of  connections  of  two  three-wire  direct-current  gen- 
erators operating  in  parallel,  125-250  volts. 


.'•Neutral  Bus 


u- •Positive  Bus 


Positive        I  Negative-'- 
0  twI'terBus-   Equalizer  Bus 
<-  -  -Switch  Switch  -  - -> 


FIG.  44. — Two  three-wire  generators  (of  the  type  having  the  balance 
coil  mounted  in  and  rotating  with  the  armature)  connected  for  parallel 
operation. 

the  negative  side.  This  insures  proper  action  of  the  inter- 
poles  at  unbalanced  load.  See  Figs.  40,  42  and  43  and  the 
text  accompanying  them. 

43.  The  Sources  of  the  Losses  in  Direct-current  Motors 
and  Generators  are  (Fig.  46):  (1)  The  resistance  of  cir- 


28 


ELECTRICAL  MACHINERY 


[ART.  44 


cults    carrying   current,    including   those  of    the    armature, 
field  coils,  interpoles  and  brush  contact;    (2)  hysteresis   and 


-  •  -  Genera  finer  Station -->K- 


-Line *» -Load- - 


FIG.  45. — Connections  for  the  parallel  operation  of  two  commutating- 
pole,  three-wire  generators. 

eddy  currents  in  the  armature-core   structure;    (3)    friction, 

including  that  of  the  commuta- 
tor and  bearings,  and  windage, 
or  the  friction  produced  by  the 
rotation  of  the  armature  in  the 
air. 

44.  Performance     Data    for 
Standard     Compound-wound 
Direct-current,     Commutating- 
pole   Generators  will  be  found 
tabulated      in      the      author's 
AMERICAN  ELECTRICIAN'S  HAND- 
BOOK.    The  efficiencies  at  vari- 
ous loads,  and  the  current  out- 
puts at   the  different  standard 
voltages    for    machines    of   the 
standard    capacities    are    there 
given. 

45.  The  Performance    Guar- 
antees on  Direct-current  Gen- 


3000 

i 

2800 

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/ 

A 

/ 

MOO 

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/ 

7700 

/ 

/ 

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1600 

P- 

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2 

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s 

in  -  ... 

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1  / 

Vr 

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r>r- 

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600 
600 
400 

y 

—  i  — 
~i  — 

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*/ 

WL 

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Mu 

7f 

£&& 

J 

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n 

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Rr 

ushCcrf* 
>  12   14  1 

>r 
0  222^ 

1      '  -^      ^ 
24681 

Horsepower  Output 


Fia.  46.  —  Losses,  individual 
and  total,  in  a  20  h.p.,  220- 
volt,  direct-current  motor,  or 
in  an  equivalent  generator. 


erators  for  reciprocating  engine  drive  are  still  made  on  the 
so-called  " normal"  basis  described  hereinunder.     It  is  not 


SEC.  1]  GENERATORS  AND  MOTORS  29 

improbable  that  all  direct-current  generators  will  ultimately 
be  rated  on  the  continuous  basis  (Art.  47a).  These  nor- 
mally rated  direct-current  generators  are  usually  guaranteed 
to  operate  continuously  at  their  full-rated  (nameplate)  kilo- 
watt outputs  with  a  temperature  rise  not  to  exceed  40 
deg.  C.  and  furthermore  they  will  operate  at  an  overload  of 
25  per  cent,  above  their  normal  or  nominal  ratings  for  two 
hours  with  a  temperature  rise  not  to  exceed  55  deg.  C. 
Direct-current  generators  for  turbine  drive  are  also  rated  on 
the  normal  basis  and  will  deliver  continuously  their  rated  kw. 
outputs  with  a  45-deg.  C.  rise  and  have  an  overload  capacity 
of  25  per  cent,  for  two  hours,  with  a  rise  not  to  exceed  55 
deg.  These  rises  are  now  all  based  on  an  ambient  (Art  476) 
temperature  of  25  deg.  C.* 

46.  Performance  Data  for  Direct -current  Motors  will  be 
found  tabulated  in  the  author's   AMERICAN  ELECTRICIAN'S 
HANDBOOK.     The  current  inputs  and  efficiencies  of  machines 
of  different  capacities  and  voltages  are  there  shown. 

47.  Performance  Guarantees  on  Direct-current  Motors  are 
now    usually  made  on  the  so-called  "normal"  basis.     How- 
ever, it  is  probable  that  all  motors  made  in  this  country  will 
shortly  be  rated  on  the  continuous   (Art.  47a)  basis.     The 
normal  rating    usually    given    these    motors    specifies    that 
they  will  operate  continuously  at  their  rated    (nameplate) 
horse-power  outputs  with  a  temperature  rise  not  to  exceed 
40  deg.  C.  and  that  they  will  operate  at  a  25  per  cent,  over- 
load for  two  hours  with  a  temperature  rise  not  to  exceed  55 
deg.  C.     The  above  rises  are  now  based  on  an  ambient  (Art. 
476)  temperature  of  25  deg.  C.* 

47a.  Continuous  Rating,  f — A  machine  thus  rated  shall  be 
able  to  operate  continuously  at  rated  output,  without  exceed- 
ing the  limitations  referred  to  in  Std.  Rule  No.  260. 

NOTE. — Rule  260  is:  "To  insure  satisfactory  results,  electrical  ma- 
chinery should  be  specified  to  conform  to  the  Institute  Standardization 

*  Note,  from  Art.  476,  that  25  deg.  C.,  though  it  is  now  used  by  practically  all 
manufacturers  for  d.  c.  machines  is  not  the  standard  recommended  by  the  A.  I.  E.  E.  It 
is  probable  that  the  standard  40  deg.  ambient  temperature  will  ultimately  be  adopted 
universally. 

t  A.  I.  E.  E.  Standardization  Rules. 


30  ELECTRICAL  MACHINERY  [ART.  47 

Rules,  in  order  that  it  shall  comply,  in  operation,  with  approved  limita- 
tions in  the  following  respects,  so  far  as  they  are  applicable:  (a) 
Operating  temperature.  (6)  Mechanical  strength,  (c)  Commutation. 
(d)  Dielectric  strength,  (e)  Insulation  resistance.  (/)  Efficiency,  (g] 
Power  factor.  (K)  Wave  shape,  (i)  Regulation. 

47b.  The  Ambient  Temperature*  is  the  temperature  of 
the  air  or  water  which,  coming  into  contact  with  the  heated 
parts  of  a  machine,  carries  off  its  heat.  (See  Arts.  47  and  256.) 

NOTE.  —  For  water-cooled  machinery,  the  standard  temperature  of 
reference  for  incoming  cooling  water  shall  be  25°  C*  (77  deg.  F),  measured 
at  the  intake  of  the  machine.  "The  standard*  ambient  temperature  of 
reference  for  air  shall  be  40  deg.  C  (104  deg.  F)." 

48.  To  Compute  the  Kilowatt  Output,  or  the  Current,  or 
Voltage  of  any  Direct-current  Generator,  it  is  merely 
necessary  to  remember  that  the  product  of  current  X  voltage 
=  watts  and  that  there  are  1,000  watts  in  a  kilowatt.  Thus: 

T?  V  7 

(2)  kw.Q  =     --  (kilowatts) 


,Q.  ™      kw.p  X  1,000  ,     u  , 

(3)  E  =  -  TJ--  -  (volts) 


,,,  T  .Q  X  1,000  , 

(4)  /  =  -      —  gr—  (amperes) 

Wherein,  kw.o  =  the  power  output  of  the  generator,  in  kilo- 
watts. E  =  the  e.m.f.,  in  volts,  which  the  generator  im- 
presses at  its  terminals  on  the  line.  I  =  the  current  impelled 
by  the  generator,  in  amperes. 

EXAMPLE.  —  What  is  the  kilowatt  output  of  a  direct-current  gen- 
erator when  it  is  impressing  500  volts  on  its  external  circuit  and  im- 
pelling a  current  of  124  amp.?  SOLUTION.  —  Substitute  in  equation  (2): 
kw.0  =  (E  X  /)  -5-  1,000  =  (500  X  124)  -=-  1,000  =  62  kw. 

EXAMPLE.  —  A  certain  220-volt  direct-current  generator  has  a  full-load 
rating  of  50  kw.  What  is  the  full-load  current  of  this  machine? 
SOLUTION.  —  Substitute  in  equation  (4):  /  =  (kw.Q  X  1,000)  -r  E  = 
(50  X  1,000)  -T-  220  =  227  amp. 

49.  To  Compute,  for  a  Direct-current  Generator,  Either 
the  Horse  -power  Input,  the  Kilowatt  Output  or  the  Efficiency 
when  the  Values  for  Any  Two  of  these  Quantities  are 

*A.  I.  E.  E.  Standardization  Rules,  December  1916. 


SEC.  1]  GENERATORS  AND  MOTORS  31 

Known  (it  being  remembered  that  there  are  746  watts  in  a 
horse-power)  the  following  formulas  may  be  used: 

(5)  h.p.i  =     --  (horse-power) 


(6)  kw.Q  =  h.p.i  X  E  X  0.746  (kilowatts) 

W  E  =  (efficiency) 


Wherein,  all  of  the  symbols  have  the  same  meanings  as  in 
Art.  50  except  that  kw.o  =  the  output  in  kilowatts  of  the 
generator. 

EXAMPLE.  —  If  a  direct-current  generator  delivering  400  kw.  has  at 
that  load  an  efficiency  of  87  per  cent.,  what  horse-power  is  then  required 
to  drive  the  machine?  SOLUTION.  —  Substitute  inequation  (5):  h.p.i  = 
kw.0  -T-  (E  X  0.746).=  400  -r-  (0.87  X  0.746)  =  400  +  0.65  =  615.4  h.p. 

50.  To  Compute,  for  a  Direct-current  Generator,  Either 
the  Horse-power  Required  to  Drive  it,  its  Voltage,  Current 
or  Efficiency  when  the  Value  of  only  One  of  these  Quantities 
is  not  Known,  one  of  the  following  formulas  may  be  used: 

EX  I 

P>i  =  E  X  746  (horse-power) 


E  = 


Wherein,  h.p.i  =  the  input  of  the  generator  in  horse-power. 
E  =  the  e.m.f.,  in  volts,  impressed  by  the  generator  on  the 
external  circuit.  /  =  the  current  impelled  in  the  external 
circuit  by  the  generator.  E  =  the  efficiency  of  the  generator 
expressed  decimally. 

EXAMPLE.  —  What  horse-power  would  be  required  at  the  pulley, 
P,  of  the  direct-current,  220-volt  generator  shown  in  Fig.  47,  when 
the  machine  was  delivering  300  amp.,  assuming  that  its  efficiency  at 
this  load  is  90  per  cent.?  SOLUTION.  —  Substitute  in  equation  (8): 
h.p.i  =  (E  X  /)  -5-  (E  X  746)  =  (220  X  300)  ^  (0.90  X  746)  =  66,000 
-f-  671.4  =  98.3  h.p.  That  is,  under  these  conditions,  98.3  h.p.  would 
have  to  be  delivered  at  the  pulley,  P,  to  pull  the  load. 


32 


ELECTRICAL  MACHINERY 


[ART.  51 


EXAMPLE.  —  What  will  be  the  efficiency  of  a  220-volt,  direct-current 
generator,  if,  when  it  is  delivering  a  current  of  108  amp.  it  requires  37.5 
h.p.  at  the  pulley  of  the  generator  to  drive  it?  SOLUTION.  —  Substitute 
in  equation  (11):  E  =  (E  X  /)  -5-  (h.p.t  X  746)  =  23,760  -j-  27,975  = 
0.849  or,  say,  85  per  cent. 


FIG.  47.  —  Example  in  figuring  size  of  steam  engine  required  to  drive  a 
direct-current  generator. 

51.  To  Find  the  Size  Engine  Required  to.  Drive  a  Direct- 
current  Generator  first  compute  the  horse-power  necessary 
to  drive  the  machine  at  full-load  by  using  equation  (8). 
Then,  select  the  engine  of  such  a  capacity  that  it  will  drive 
the  generator,  the  overload  capacity  of  the  engine  and  the 
generator,  if  there  is  such,  being  considered  in  each  case.  If 
the  generator  is  rated  on  the  maximum  or  continuous  basis, 
due  allowance  must  be  made  for  this  in  selecting  the  engine. 
If  the  generator  is  to  be  belt-driven  an  allowance  for  a  power 
loss  of  from  2  to  5  per  cent,  in  the  belt  drive  should  also  be 
made. 


NOTE.  —  For  a  direct-current  generator,  0.746  X  engine  brake  horse- 
power =  the  kilowatt  capacity  of  the  engine,  which  would  be  the  power 
input,  in  kilowatts,  to  the  generator.  Multiplying  this  quan- 
tity by  the  assumed  efficiency  of  the  generator  will  give  the  kilo- 
watts output  of  the  generator.  Where  this  generator  efficiency  is 
not  known  it  may  be  assumed  to  be  90  per  cent.,  which  is  an  average 
value.  Thus,  for  a  working  approximation:  0.90  X  0.746  X  brake 
horse-power  =>  the  kilowatt  rating  of  generator.  A  complete  table 
of  the  efficiencies  of  direct-current  generators  of  various  capacities 
will  be  found  in  the  author's  AMERICAN  ELECTRICIAN'S  HANDBOOK. 
In  the  discussion  immediately  preceding,  it  has  been  assumed  that 
the  brake  horse-power  of  the  engine  in  question  is  known.  If  it  is 
not,  a  sufficiently  accurate  expression  for  brake  horse-power  may  be 
obtained  by  multiplying  the  indicated  horse-power  of  the  engine  by 
its  efficiency,  an  average  value  for  which  may  be  taken  as  90  per  cent. 


SEC.  1] 


GENERATORS  AND  MOTORS 


33 


EXAMPLE. — What  size  internal-combustion  engine,  that  is,  gas, 
gasoline  or  oil  engine,  should  be  used  to  drive  the  50-kw.  normally 
rated,  direct-current  generator  shown  in  Fig.  48?  SOLUTION. — Normally 
rated  generators  usually  have  an  overload  capacity  of  25  per  cent, 
for  two  hours.  Hence,  the  maximum  power  that  this  machine  could 
develop  for  any  considerable  length  of  time  would  be:  50  kw.  X  1.25 
=  62.5  kw.  Now,  from  equation  (5),  h.p.i  =  kw.0  -i-  (E  X  0.746)  = 
62.5  -5-  (0.84  X  0.746)  =  99.6  h.p.  That  is,  99.6  h.p.  would  be  re- 
quired in  mechanical  power  at  the  generator  pulley,  to  produce  62.5  kw. 
of  electrical  power  at  the  generator  terminals.  Assuming  a  belt 


50  kw.  Normally  Rated 
Direct-Current  Generator 
(Efficiency  =84%) 

Ammeter 


'  r     •     f -•  ..._-,,. 

FIG.  48. — Example  in  computing  size  of  gas-engine  required  to  drive  a 
direct-current  generator. 

loss  of  5  per  cent,  there  would  be  required  at  the  engine  flywheel: 
99.6  h.p.  X  1.05  =  104.6  h.p.  Hence  the  engine,  E,  should  have  a 
rating  of  at  least  104.6  h.p.  if  the  full  capacity  of  the  generator  is  to  be 
developed.  Normally,  combustion  engines  have  little,  if  any,  overload 
capacity.  In  practice,  a  100-h.p.  internal-combustion  engine  would 
probably  be  used  for  the  application  in  this  discussion. 

EXAMPLE. — If  the  generator  shown  in  Fig.  47  had  a  normal  full-load 
rating  of  100  kw.  and  an  efficiency  of  90  per  cent.,  there  would  be 
required  to  drive  it  at  full-load:  100  kw.  +  0.90  =  111  kw.  Now, 
the  horse-power  equivalent  of  111  kw.  is:  111  kw.  -i-  0.746  =  149  h.p. 
Then,  if  a  belt  loss  of  3  per  cent,  be  assumed,  the  engine  would  have 
to  develop, when  full-load  is  on  the  generator,  149  h.p.  -5-  0.97  =  154 
h.p.  That  is,  an  engine  rated  at  about  154  brake  horse-power  should 
be  used  to  drive  this  machine.  The  overload  capacity  of  a  steam 
engine,  as  engines  are  usually  rated,  is  about  equivalent  to  the  overload 
capacity  of  a  normally  rated,  direct-current  generator. 

52.  Direct -current  Motors  are  of  the  same  construction 
as  direct-current  generators  having  the  same  types  of 
windings.  That  is,  the  construction  of  a  shunt-wound  motor 
is  the  same  as  that  of  a  shunt-wound  generator,  a  compound 

3 


34  ELECTRICAL  MACHINERY  [ART.  53 

wound  motor  the  same  as  a  compound-wound  generator, 
and  so  on.  In  fact,  the  electrical  machinery  manufacturers 
frequently  use  precisely  the  same  direct-current  machines  for 
generators  as  for  motors,  merely  changing  the  nameplates 
on  them,  as  occasion  requires,  before  shipping  them. 
Hence,  it  follows  that  much  of  the  information  relating  to 
direct-current  generators  given  in  the  preceding  pages  will 
apply  directly  to  direct-current  motors. 

53.  Performance  Data  for  Direct-current  Motors  (stand- 
ard efcciencies,  ratings,  speeds  and  similar  information)  will  be 
found  tabulated  in  the  author's  AMERICAN  ELECTRICIAN'S 
HANDBOOK. 

64.  The    Output    or    Horse -power   of    a    Direct-current 
Motor,  or  any  other  motor  for  that  matter,  is  proportional 
to  the  product  of  its  torque  and  its  speed. 

65.  The  Torque  of  a  Direct-current  Motor  is  proportional 
to  its  effective  magnetic  field,  the  number  of  armature  con- 
ductors, and  the  current  or  amperes  flowing  in  the  armature. 

56.  The  Speed  of  a  Direct-current  Motor  is  proportional  to 
the  following  ratio: — [(voltage  impressed  on  motor  terminals)  — 
(volts  drop  due  to  armature-circuit  resistance)]  -~  [(number  of 
armature  conductors)  X  (effective  magnetic  field)]. 

57.  Commutation  of  Direct-current  Motors. — Before  com- 
mutating-pole    motors     were    manufactured,    commutation 
determined  the  overload  capacity  of  direct-current  motors. 
A  modern  commutating-pole  motor  will  carry  100  to  125  per 
cent,  overload,  that  is,  2  to  2.25  times  the  normal  or  full- 
load  without  sparking.     The  heating  of  the  machine  under 
load  is  now  a  more  important  factor  than  formerly 

58.  Direct -current  Motors  of  the  Series,  Shunt  and  Com- 
pound Types  Have  Different  Speed  Characteristics.* — That 
is,  as  the  load  on  an  unloaded  motor  is  increased,  its  speed  may 
decrease  slowly  or  rapidly  or  remain  practically  constant,  de- 
pending on  how  the  motor  is  wound  (series,  shunt  or  com- 
pound) and  on  its  design.     Motors  of  all  types  should  develop 

•See  article,  "Speed  Characteristics  of  Direct-current  Motors,"  Alan  M.  Bennett, 
POWER,  Jan.  26,  1915. 


SEC.  1] 


GENERATORS  AND  MOTORS 


35 


their  rated  speeds  at  full-load  after  they  have  been  in  opera- 
tion for  a  sufficient  period  to  attain  their  maximum  full-load 
temperatures,  hence  the  term  "speed  of  a  motor"  means  the 
full-load  speed  under  these  conditions.  Variations  from  the 
rated  speed  may  be  conveniently  considered  at  two  periods 
in  the  operation  of  the  motor,  namely:  (1)  at  the  time  the 
motor  is  started  cold;  and  (2)  at  no-load,  but  after  the  motor 
has  attained  its  working  temperature.  The  amount  by  which 
the  speed  under  the  first  condition  differs  from  the  rated  speed 
is  known  as  the  speed  variation  of  the  motor.  It  is  sometimes 
referred  to  as  the  variation  from  "cold  to  hot"  at  full-load. 
The  change  from  rated  speed  at  no-load,  but  at  working  tem- 
perature, that  is,  the  difference  between  full-load  speed  and 
no-load  speed,  is  known  as  the  speed Regulation  of  the  motor. 
Speed  regulation  is  usually 
expressed  as  a  percentage  of 
the  full-load  speed.  For  each 
condition  the  departure  from 
rated  speed  is  expressed  as  a 
percentage  of  the  rated  speed. 
As  would  be  inferred,  both  the 
speed  variation  and  the  regu- 
lation may  differ  for  motors 
of  the  same  general  class  and 
the  same  rating  because  these 


900 


20   22 


FIG.  49. — Graphs  showing  speed 
characteristics  of  shunt  motors  of 
different  designs.  Curve  I  from  a 
20-h.p.  four  pole,  230-volt,  900 
r.p.m.  Motor.  Curve  II  from  a 
50-h.p.  four-pole,  230-volt,  576 
r.p.m.  motor.  Curve  III  from  a 
4-h.p.  four-pole,  230-volt,  235 
r.p.m.  motor.  (Electric  Journal.) 


characteristics  are  determined 
by  the  designs  of  the  ma- 
chines. See  Fig.  49.  It  can 
be  shown  that  the  speed  of  a 
motor  varies  directly  as  the 
impressed  voltage  minus  the  I 
X  R  drop  in  the  armature  circuit  and  inversely  as  the  flux. 
In  this  expression,  I  =  the  armature  current,  and  R  =  the 
resistance  of  the  motor  windings  and  the  brush  contacts 
which  are  in  series  with  the  armature.  It  follows  that  any 
condition  affecting  the  operation  of  a  motor  which  tends  to 
increase  either  this  I  X  R  drop  or  the  flux,  will  lower  the 
speed  of  the  motor.  Conversely,  a  decrease  in  either  flux  or 


36 


ELECTRICAL  MACHINERY 


[ART.  58 


I  X  R  drop  will  raise  the  speed.     How  this  I  X  R  drop  and 
the  flux  vary  in  motors  with  series,  shunt  and  compound  wind- 


**•  900 

I" 

3700 


500 

I400 
^300 

^200 


too 


\ 


•0     25   50   75    100  125  150  175  TOO  125  250  275 
Amperes 

FIG.  50. — Graph  indicating  the  torque  and  speed  characteristics  of  a 
series-wound  direct-current  motor.  (A  37.5  h.p.,  4-pole,  230- volt,  540 
r.p.m.  machine.) 

Series  Wound  t).C.Motor  -£ 

S                     220  Volts                   „  *- 

0.  Cotpacity-20H.Rfor4Hourwith40C.  E 
Rise  by  Thermometer. 


1400 

\  i 

......  "^ 

\  \ 

0     1^00- 

\\\ 

^ 

*  1 

\\\\ 

2 

*G     i?nn 

\\\ 

C^i 

I..S--2 

\\\ 

^c 

S-      * 

i     "J    1  1  no 

\\ 

4 

o         iL 

\ 

< 

H:        '£ 

\ 

s  ^ 

.  J3  .  ...  1 

\\ 

\  r 

\\ 

5 

^ 

V  ^  450  — 

±fc 

4Q—  60-&00- 

^ 

;; 

7^            V           ^ 

2-  400-  -0 

E 

c 

f          A     Sj 

^     *ZCA 

1 

^  8  •* 

S   Z             S    S 

x      JbU 

^  . 

^  ^       r    X* 

•rnrt 

v 

/       /J  / 

P 

A  i 

OCA 

C9U 

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*  200 

/  /  /z 

-    150 

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^  y  o! 

b 

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100 

( 

K 

^ 

Cft 

f\Jf 

DV 

FIG.  51.- 


S  S  I  g  1  S 

Amperes 
-Performance  curves  for  a  direct-current  series-wound  motor. 


ings,  and  thereby  affect  the  speed,  and  why  they  vary  in  ma- 
chines with  these  different  types  of  windings  will  be  briefly 


SEC.  1] 


GENERATORS  AND  MOTORS 


37 


discussed  in  following  articles.  Speed  variation  and  control 
of  series-,  shunt-,  and  compound-wound  motors  are  further 
treated  in  Arts.  121,  124,  and  125. 

69.  The  Speed -torque  Characteristics  of  a  Series-wound 
Direct-current  Motor  are  shown  graphically  by  the  graph  of 
Fig.  50.  It  will  be  noted  that  the  speed  decreases  rapidly 
as  the  load  on  (amperes 
taken  by)  the  motor  in- 
creases. Furthermore,  the 
torque  developed  by  the 
motor  increases  as  the  cur- 
rent increases.  The  signifi- 
cance of  these  graphs  is 
further  explained  in  the  ex- 
ample following  Art.  61, 
which  discusses  the  graphs  of 
Fig.  51;  see  also  the  compar- 
ative graphs,  A,  B,  C  and  D, 
of  Fig.  52. 

Some  of  the  variation  in 
the  speed  of  a  series  motor  is 
due  to  the  I  X  R  drop  in  its 
brush  contacts,  armature  and 
series  winding.  However, 
most  of  the  variation  is  due 
to  the  fact  that  the  flux — 


1000 


40 


FIG.  52. — Graphs  comparing  the 
speed-load  characteristics  of  10 
h.p.  shunt,  series-  and  compound- 
wound,  direct-current  motors. 


which  is  generated  by  the  series-field  winding  which  carries 
line  current — increases  almost  directly  with  the  load.  The 
I  X  R  drop  increases  somewhat  because  R,  the  resistance 
of  the  armature,  remains  practically  constant  but  /,  which 
with  a  series  motor  is  the  line  current,  increases  directly  with 
the  load.  The  fact  that  as  the  motor  is  loaded  its  windings 
become  hotter  and  hence  have  a  higher  resistance  than  at 
no-load,  also  has  the  same  effect.  Hence,  this  heat  effect 
would  tend  to  increase  the  I  X  R  drop  and  lower  the  speed, 
but  in  practice  it  is  of  little  consequence. 

Note  that  with  the  series  motor  the  increase  of  temperature 
has  an  ultimate  effect  opposite  to  that  which  obtains  with  a 


38  ELECTRICAL  MACHINERY  [ART.  60 

shunt  motor  (Art.  63),  for  which  increased  temperature  re- 
sults in  increased  speed.  The  speed  variation  in  an  average 
series  motor  due  to  increased  temperature  is,  at  full-load, 
approximately  2  per  cent.  This  is  almost  negligible,  especially 
as  compared  with  the  change  in  speed  of  the  motor  due  to 
the  change  in  its  series-field  current — or  load.  While  the 
change  in  speed  produced  by  the  /  X  R  drop  due  to  a  change 
of  load  is  greater  with  a  series  than  with  a  shunt  motor  (be- 
cause as  the  load  increases,  the  I  X  R  drop  in  the  series-field 
exciting  winding  is  increased),  it  is  small  as  compared  with 
the  change  in  speed  due  to  flux  variation.  In  fact  it  can  be 
shown  that  the  speed  of  a  series  motor,  particularly  when  it 
is  operating  below  the  saturation  point  of  the  field,  will  vary 
almost  exactly  inversely  with  the  change  of  flux.  This  proves 
that  the  I  X  R  drop  is  of  little  consequence.  Fig.  52  shows 
graphically  how  much  more  rapidly  the  speed  of  a  series  motor 
changes  with  the  load  than  does  that  of  a  shunt  or  lightly 
compounded  motor. 

60.  Theoretically,  a  Series  Motor  Will  Run  at  an  Infinite 
Speed  when  Pulling  No-load. — Practically,  one  of  these  ma- 
chines if  connected  across  a  source  of  constant  e.m.f.   and 
operated  without  a  load  will  "speed  up"  until  it  bursts  its 
armature  band  wires  and  throws  the  armature  winding  out 
of  the  slots  by  centrifugal  force.     That  is,  unloaded  series 
motors  "run  away." 

61.  The  Method  of  Reading  Direct-current,  Series-wound, 
Motor  Performance  Graphs*  is  illustrated  by  Fig.  51.     To 
determine  the  nominal  full-load  characteristics  of  a  motor  hav- 
ing the  curves  illustrated,  proceed  as  follows: 

EXAMPLE. — First  find  the  point  at  which  the  brake  horse-power  curve 
intersects  the  20-h.p.  horizontal  line  (20  h.p.  being  the  nominal  rating 
of  the  motor).  This  point  is  on  the  80-amp.  vertical  line.  The  char- 
acteristics of  the  motor  at  nominal  full-load  will  be  those  denoted  by 
the  curves  at  the  points  where  the  80-amp.  vertical  line  intersects  them. 
Thus  the  motor  will  take  80  amp.;  the  torque  will  be  175  Ib.  at  1-ft. 
radius;  the  speed  will  be  about  600  r.p.m. ;  the  efficiency  84  pel  cent. ;  and 
it  will  take  35  min.  continuous  operation  for  the  motor  temperature 
to  rise  to  40  deg.  C. 

*  WKSTINQHOUSE  PUBLICATION. 


SEC.  1]  GENERATORS  AND  MOTORS  39 

If  the  motor  is,  for  example,  required  to  develop  250  Ib.  torque,  its 
characteristics  under  this  condition  are  found  in  a  similar  manner,  the 
proper  ampere  vertical  line  being  that  which  intersects  the  point  where 
the  250-lb.  torque  horizontal  line  and  the  torque  curve  intersect — in 
this  case  105  amp. 

An  inspection  of  the  curves  shows  that  the  torque  is  maximum  at  start- 
ing and  decreases  as  the  speed  increases.  The  higher  the  torque,  the 
greater  the  current  required  and  the  more  rapidly  the  motor  temperature 
will  rise.  Moreover  it  is  evident  that  the  rating  of  20  h.p.  for  this  motor 
is  arbitrary  only.  This  rating  is  based  on  the  power  developed  with  a 
temperature  rise  of  40  deg.  C.  in  one-half  hour's  continuous  operation; 
if  the  motor  is  operated  continuously  for  periods  of  one  hour,  it  could 
not  be  conservatively  rated  above  15  h.p.,  while  if  the  periods  of  opera- 
tion are  very  short  and  intervals  of  rest  long,  a  rating  of  higher  than  20 
h.p.  would  be  satisfactory. 

62.  The  Proper  Connections  for  a  Shunt  Motor  are  as  shown 
in  Fig.  53.  The  field  B  is  connected  as  shown,  so  that,  when 
the  main  switch  D  is  closed,  it  becomes  excited  before  the  arma- 
ture circuit  switch  at  E  is  closed.  Thus,  when  the  motor 


Switch 

Shunt  Field 

B   I 

3S 

OE                                             -<^-      1 

Starting 
Box--} 

T  n 

s< 

Armature' 

-00<>0 

v: 

FIG.  53. — Control  apparatus  connections  for  a  shunt  motor. 

armature  has  current  admitted  to  it  by  the  closing  of  switch 
at  E  and  by  the  operation  of  starting  rheostat  A,  the  field  is 
already  on,  and  the  full  torque  of  the  motor  is  thereby  ob- 
tained at  starting.  The  torque  of  a  motor  is  equal  to  the 
product  of  flux  per  pole,  the  ampere-turns  on  the  armature, 
and  the  number  of  poles.  Hence,  if  the  full  field  is  not  on 
the  motor  at  the  instant  of  starting,  full  torque  will  not  be 
obtained  at  that  instant.  See  also  Figs.  83,  86,  and  104  for 
connections  for  starting  equipment  for  direct-current  shunt 
motors. 

63.  The  Speed  Characteristic  for  a  Shunt  Motor  is  shown 
in  Fig.  54.     See  also  Fig.  55.     It  will  be  noted  that  the  speed 


40 


ELECTRICAL  MACHINERY 


[ART.  63 


remains  almost  constant  from  no-load  to  full-load  and  that 

the  torque  increases  almost 
directly  with  the  load.  With 
a  shunt  motor  on  a  constant- 
potential  circuit,  since  the 
shunt-field  winding,  Fig.  9,  is 
connected  directly  across  the 
constant-voltage  supply 
source,  the  main  flux  remains 
almost  constant  at  all  loads. 
However,  as  the  motor  heats, 
the  resistance  of  the  shunt- 
field  winding  will  increase, 
which  decreases  correspond- 
ingly the  shunt-field  exciting  current  and  the  main  flux.  But 
this  flux  is  decreased  slightly,  which  tends  to  make  the 

60 


-  -        40-60        80        wu 
i*er  Cent  of  Full-Load  Current 

FIG.  54. — Typical   speed-torque 
graph  of  a  shunt-wound  motor. 


20 


0     0 


50  W 

FIG.  55. — Typical  performance  graphs  for  a  shunt  motor. 


30  40 

Amperes  Input 


motor  " speed  up."     Tests  show  that  the  speed  increase  due 
to  increase  in  temperature  of  the  shunt-field  winding  is,  froni 


SEC.  1] 


GENERATORS  AND  MOTORS 


41 


"cold  to  hot"  at  full-load,  from  4  to  8  per  cent,  for  commercial 
motors.  But  this  tendency  toward  an  increase  in  speed,  due 
to  increased  resistance  of  the  field  winding,  has  little  effect  in 
practice  because  the  I  X  R  drop  in  the  motor  armature  as 
the  load  comes  on  the  motor  more  than  offsets  its  temperature 
effect.  Obviously,  as  the  load  on  the  motor  increases,  the 
I  X  R  drop  in  the  armature  circuit  increases  in  proportion  and 
it  is  due  to  this  that  the  speed  of  a  shunt  motor  decreases  from 
no-load  to  full-load  as  shown  in  Figs.  54  and  52.  Tests  indi- 
cate that  the  speed  regulation  of  shunt  motors  as  they  are  or- 
dinarily manufactured  ranges  from  about  4  to  6  per  cent. 

64.  By  Shifting  the  Brushes  the  Tendency  of  a  Shunt  Motor 
to  Decrease  in  Speed  as  its  Load  Increases  can  be  Partially 
Offset  in  non-commutating  pole  motors.     The  reverse  is  also 
true.     To  effect  this  result,  the  brushes  should  be  shifted 
"backward."     By  this  procedure,  a  portion  of  the  flux  due 
to  the  armature  ampere-turns  is  caused  to  oppose  the  main 
flux.     This  weakens  the  main  flux  with  the  result  that  the 
speed  of  the  motor  is  increased. 

However,  brush  shifting  can  be 
utilized  to  increase  the  speed 
of  a  motor  only  within  the 
limits  wherein  sparking  at  the 
commutator  will  not  be  ex- 
cessive. 

65.  The    Speed    Regulation 
of   a   Commutating-pole  Shunt 
Motor   is   affected    by  the   ac- 
tion of  the  commutating  poles. 
These   commutating  poles  pro- 
duce a  weakening  effect  on  the 

main-field  flux  similar  to  that  produced  by  the  current  in  the 
armature.  The  result  may  be  an  increase  in  the  speed  with 
the  load  (Fig.  56).  This  effect  is  particularly  noticeable 
with  adjustable-speed  motors  when  they  are  being  operated 
at  high  speeds — that  is,  with  weak  fields.  Then,  due  to  the 
action  of  commutating  poles,  a  motor  may  rotate  at  a 
higher  speed  at  full-load  than  at  no-load,  as  shown  at  AB, 


1300 


f  1100 
31000 
1900 


400 


SpeecfJOO 


15 

Ampere^ 


FIG.  56. — Speed  graphs  for  a 
7^-h.p.,  commutating-pole,  ad- 
justable-speed shunt  motor. 


42 


ELECTRICAL  MACHINERY 


[ART.  66 


Fig.  56.  In  commutating-pole  motors,  brush  shifting  is  not 
feasible,  hence,  even  if  it  were  necessary,  this  expedient 
could  not  be  utilized  to  maintain  the  motor  speed  constant. 
In  the  graph  of  Fig.  56,  it  will  be  noted  that  the  motor 
has  a  normal  speed  of  400  r.p.m.  which  can  be,  by  means  of 
shunt-field  control,  increased  to  1,200  r.p.m.  At  the  normal 
speed,  CD,  the  regulation  is  within  1%  per  cent.,  the  speed 
remaining  practically  constant.  At  the  higher  speed,  AB, 
the  revolutions  per  minute  actually  increase  about  2  per  cent. 
or  25  r.p.m.  from  no-load  to  full-load. 

66.  The  Speed  Characteristics  of  Shunt  Motors  of  Different 
Designs  are  shown  in  Fig.  49,  from  which  it  is  evident  that 
the  properties,  in  so  far  as  speed  is  concerned,  of  the  shunt 
motor  may  be  subject  to  considerable  variation  in  machines 
of  various  designs. 


Speed 

Adjusting  Nu\ 


Hand  Wheel  for 
Speed  Adjustment ^ 


Lever. 

Lever 

Fulcrum- 

Pin 


Commutator 
End  Yoke—' 


Commutating 
Pole 


Main  Field 
Pole 


FIG.  56a. — Partial  sectional  elevation  of  the  armature-shifting-design, 
Reliance  adjustable-speed  motor. 

66a.  The  Reliance  Adjustable-speed  Motor  employs  a 
novel  method  of  varying  the  field  which  the  armature  inductors 
cut  whereby  the  speed  of  the  motor  is  changed.  Typical 
designs  for  a  motor  of  this  type  are  shown  in  Figs.  56a  and 
566.  The  motor  is,  in  essence,  as  shown  in  Fig.  56a,  a  shunt- 
wound,  direct-current,  commutating-pole  motor  of  the  usual 
design.  However,  there  are  these  important  differences:  (1) 


SEC.  1]  GENERATORS  AND  MOTORS  43 

The  armature,  A,  together  with  its  shaft,  may  be  shifted 
longitudinally  by  a  hand  wheel,  H,  so  that  the  armature  may 
be  made  to  lie  wholly  or  only  partially,  at  the  will  of  the 
operator,  under  the  influence  of  the  main-field-pole  (P)  flux. 
(2)  The  commutating  poles,  C  (Fig.  56a)  are  located  at  the 
commutator  ends  of  the  main  poles. 

When  the  handwheel,  H,  is  turned  to  such  a  position  that  the 
armature  lies  wholly  between  the  main  poles  then  the  armature 


Hanoi  Wheel  for 
Speed  A 


Commutator 
Enei  Yoke 


BoltHoictina'' 
C  omrnuterting 


FIG.  566. — Reliance  adjustable-speed  motor,  type  AS  armature-shifting 
design.     (Reliance  Electric  &  Engineering  Co.,  Cleveland,  Ohio.) 


inductors  are  cutting  a  maximum  of  flux  and  the  motor  will 
rotate  at  its  slowest  speed.  But,  if  the  handwheel  is  turned 
until  the  armature  lies,  insofar  as  it  can  be  made  to  do  so,  out- 
side of  the  influence  of  the  main-field  poles,  then  the  armature 
inductors  cut  a  minimum  flux  and  the  armature  will  rotate 
at  its  greatest  speed.  Obviously,  an  infinite  number  of  run- 
ning speeds  between  the  maximum  and  minimum  are  obtain- 
able. Speed  variations  of  10  to  1  are,  it  is  claimed,  satis- 
factorily attained  in  practice. 


44  ELECTRICAL  MACHINERY  [ART.  67 

It  is  apparent  that  in  a  motor  of  this  type  the  main  fields 
are  always  saturated  because  the  effective  area  or  cross  section 
of  iron  which  carries  flux  decreases  as  the  armature  is  shifted 
laterally.  The  result  is  that  the  field  distortion  is  little  greater 
when  the  armature  is  rotating  at  a  high  speed  and  carrying 
full-load  current  than  when  it  is  operating  under  full-load 
at  low  speed  with  the  armature  directly  under  the  main  poles, 
where  maximum  flux  cuts  its  inductors.  A  variable-speed 
motor  of  this  type  has  a  number  of  attractive  features.  Pos- 
sibly the  most  important  is  the  simplicity  of  the  entire  arrange- 
ment and  the  ease  of  installation,  inasmuch  as  no  speed-ad- 
justing rheostat  and  the  relatively  complicated  wiring  incident 
thereto  is  necessary.  Motors  of  this  type  are  reversible  when 
equipped  with  reversing  type  starters. 

The  motor  exhibits  the  usual  adjustable-speed,  shunt- 
motor  characteristics.  That  is,  when  adjusted  for  one  given 
speed,  it  will  rotate  at  that  constant  speed  under  a  variable 
load.  The  decrease  in  speed — the  motor  being  adjusted  for 
operation  at  some  certain  speed — from  no-load  to  full-load 
is  said  to  be  very  small  and  to  compare  favorably  with  that 
of  the  best  constant-speed-motor  practice.  The  United 
States  Government,  American  Steel  &  Wire  Co.,  Illinois  Steel 
Co.,  Pennsylvania  Railroad  and  large  users  have  purchased 
motors  of  this  type.  It  is  apparent  that  they  are  well  adapted 
for  individual  drive  for  machine  tools  where  an  adjustable- 
speed  motor  is  necessary  and  it  is  in  this  service  that  they 
have  found  their  widest  application. 

67.  The  Speed  Characteristics  of  a  Compound-wound, 
Direct-current  Motor  (since  a  motor  of  this  type  has  both 
shunt-  and  series-field  windings)  partake,  of  the  character- 
istics of  both  shunt  and  series  motors.  Fig.  57  shows 
typical  performance  graphs  of  one  of  these  machines.  As 
is  evident  from  the  graphs  B  and  C  of  Fig.  52,  a  compound- 
wound  motor  may,  in  so  far  as  the  speed  characteristics 
are  concerned,  be  made  to  resemble  a  series  or  shunt  motor, 
depending  upon  the  percentage  of  the  field  flux  due  to  the 
series  and  the  shunt  windings,  respectively.  The  motor 
of  graph  B  (Fig.  52)  has  a  very  poor  speed  regulation, 


SEC.  1] 


GENERATORS  AND  MOTORS 


45 


while  that  of  graph  C  has  a  fairly  good  regulation — about 
12  per  cent. 

68.  Compound-wound  Motors  may  be  Either  Differential 
or  Cumulative  (Fig.  21)  as  may  compound- wound  generators. 
The  cumulative  compound-wound  motor  has  its  series  field 
so  connected  that  it  "  assists "  the  shunt  winding.  Thus,  the 
main  field  of  a  motor  of  this  type  is  strengthened  as  the 
load  increases.  The  result  is  that  some  of  the  properties  of  a 
series  motor — namely,  powerful  starting  torque  and  rapid 
acceleration— are  obtained.  But  when  the  series  winding  is 


20     20 


16      16 


20  30 

Amperes  Input 

FIG.  57. — Typical  performance  graphs  for  a  compound-wound  motor. 

differentially  connected  (Fig.  21,  II),  it  "opposes"  the 
shunt-field  winding.  Such  an  arrangement  would  tend  to 
compensate  for  the  /  X  R  drop  from  no-load  to  full-load  and 
render  the  motor  a  constant-speed  machine.  That  is,  as 
the  load  increases,  the  effect  of  the  flux  of  the  shunt  field  is 
decreased  by  the  action  of  the  series  field.  The  tendency 
is  then  for  the  speed,  instead  of  decreasing  due  to  the  I  X  R 
drop,  to  remain  constant.  Differentially  wound  motors  are 
seldom  applied  because  of  operating  disadvantages,  chief 
among  which  are  low  starting  torque  and  inability  to  success- 
fully handle  overloads. 


46  ELECTRICAL  MACHINERY  [ART.  69 

69.  A  Comparison  of  the  Speed  Characteristics  of  Shunt, 
Series-  and  Compound -wound  Motors  is  given  graphically 
in  Fig.  52.     This  comparison  is  on  the   basis    of  motors  of 
the  same  general   construction  and  design  but  having  wind- 
ings of  the  different  types. 

70.  The  Effect  of  Commutating  Poles  on  the  Speed  Regula- 
tion of  Motors.* — At    overloads    the    effect    on    non-com- 
mutating  pole  motors  is  a  decrease"  in  speed  proportional  to 
the  load;  but  on    commutating-pole    motors    the    speed    in 
many  cases  tends  to  increase  between  full-load  and  100  per 
cent,  overload.     Commutating-pole   motors   will,    therefore, 
have  approximately  the  same  speed  at  twice  full-load  as  at 
full-load.     If  the  effect   of   the   commutating   poles   is   too 
strong,  the  tendency  is  to  make  a  commutating-pole  motor 
oscillate  in  speed.     This  speed  oscillation  will  cause  a  similar 
variation  of  armature  current   of   gradually   increasing   in- 
tensity, until  something  gives  way;  a  fuse  will  blow,   a  cir- 
cuit-breaker open  or  the  motor  will  be  injured  by  "bucking 
over,"  that  is,    flashing  across  brushes,  or  burning  out  the 
armature. 

There  is  a  relation  between  speed  regulation  and  stability. 
A  commutating-pole  motor  can  be  designed  to  be  stable  at 
over-loads.  This  will  increase  the  drop  in  speed.  Better 
speed  regulation  makes  stability  less  certain.  Reliable  de- 
signers of  this  type  of  motor  strike  a  happy  medium  be- 
tween these  two  factors  and  the  commercial  result  is  that 
in  most  cases  these  motors  can  be  safely  operated  on  in- 
termittent loads  where  the  maximum  load  is  twice  the 
rated  load. 

i  A  large  reduction  in  speed,  insuring  a  stable  motor,  is  an 
advantage  in  machine-tool  applications.  It  often  occurs  when 
long,  continuous  cuts  are  taken,  that  on  one  part  of  a  casting 
the  depth  of  cut  is  greater  than  on  another,  due  to  irregu- 
larities in  casting.  When  cutting  through  the  heavy  part 
the  speed  should  be  reduced,  thus  protecting  the  cutting  tools 
and  machine  tool  as  well  as  the  work.  For  this  reason  ad- 

*  AMERICAN  MACHINIST,  Sept.  26,  1912. 


SEC.  1]  . 


GENERATORS  AND  MOTORS 


47 


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Multiple  voltage,  three-w 
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Multiple  voltage,  four-wire 

Motor-generator  system. 
Boost-and-retard  system. 
Teaser  system. 

Standard  motors. 
Compensated  motors. 
Commutating-pole  motors. 

Moving  poles  radially. 
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(Used  with  series  motors.) 

(Double-commutator  moto 
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48 


ELECTRICAL  MACHINERY 


[ART.  72 


justable-speed  motors,   with  a  speed  reduction  as  high  as 
25  per  cent.,  can  be  used  to  advantage. 

72.  To  Compute  Either  the  Kilowatt  Input,  Horse -power 
Output,  Efficiency,  Impressed  Voltage  or  Current  of  any  Di- 
rect-current Motor,  the  other  quantities  being  known,  one  of 
the  following  formulas  may  be  used : 

Kwi  X  E       E  X  I  X  E 
(12)  h.p.0  =      -A  „       =      n.  Ann  (horse-power) 


(13) 


E  = 


74.6 
h.p.0  X  74.6 


74,600 


X  74,600 


E  XI 


(efficiency  per  cent.) 


'-Direct'- Current 
Motor 
(Efficiency is  86%) 


L-Yoltmeter 
Reaa/s  220  Volts 

Ammeter 
~~ -Reads  80  Amp 

FIG.  58. — Example  in  computing  the  horse-power  output  of  a  direct- 
current  motor. 


(14) 
(15) 
(16) 


Kwt 
E 


X  74.6 


h.p.0  X  74,600 
EX/ 

h.p.0  X  74,600 
EXE 


(kilowatts) 

(volts) 

(amperes) 


Wherein,  h.p.0  =  power  output  of  the  motor,  in  horse-power. 
=  power  input  to  the  motor,  in  kilowatts.   E  =  efficiency 


SEC.  1] 


GENERATORS  AND  MOTORS 


49 


of  the  motor,  in  per  cent.,  at  the  output  h.p.0.  E  =  the  e.m.f. 
impressed  on  the  motor,  in  volts.  /  =  the  current  taken  by 
the  motor,  in  amperes,  for  the  output  h.p.0. 

EXAMPLE. — What  horse-power  will  the  direct  current-motor,  M,  of 
Fig.  58 — which  has,  under  the  conditions  of  this  example,  an  efficiency 
of  86  per  cent. — deliver  to  the  line  shaft,  L,  if  the  e.m.f.  impressed 
across  its  terminals  is  220  volts,  and  it  is  taking  80  amp.?  Assume 
that  the  loss  in  the  belt  drive,  B,  is  3  per  cent.  SOLUTION. — To  ascer- 
tain the  horse-power  delivered  by  the  motor  at  its  pulley,  P,  substitute 
in  equation  (12) :  h.p.0  =  (E  X  I  X  E)  -h  74,600  =  (220  X  80  X  86)  -=- 
4,600  =  1,513,600  -s-  74,600  =  20.3  h.p.,  which  is  the  power  delivered 
at  P.  Because  of  the  belt  loss,  the 
power  delivered  to  the  line  shaft  L, 
would  be  3  per  cent,  less  than  this, 
or,  0.97  X  20.3  =  19.7  h.p. 

EXAMPLE. — What  is  the  efficiency 
of  the  direct-current  motor  of  Fig. 
59,  under  the  test  conditions  there 


Pulley  Driving  Logct- 


'  'Direc  t-  Current 
Motor 


Motor  is  /// 

Delivering        /y  Direct-Current 
3?  hp  at  Motor//''  Motor 
Pulley,          //'Efficiency: 


FIG.  59.  PIG.  60. 

FIG.  59. — Example  in  computing  the  efficiency  of  a  direct-current 
motor  when  its  horse-power  output,  its  current  input  and  its  voltage 
are  known. 

FIG.  60. — Example  in  finding  current  taken  by  a  motor,  its  horse-power 
output,  voltage  and  efficiency  being  known. 

specified?  The  power  output  as  measured  by  the  Prony  brake  is  16.2 
h.p.  The  impressed  e.m.f.  is  220  volts  and  the  current  taken  by  the 
motor  is  64  amp.  SOLUTION. — Substitute  in  equation  (13) :  E  =  (h.p.0 
X  74,600)  ^  (E  X  I)  =  (16.2  X  74,600)  ^  (220  X  64)  =  1,208,520 
-r-  14,080  =  85.8.  Hence,  the  efficiency  of  this  motor  under  the  con- 
ditions illustrated  is  85.8  per  cent. 

EXAMPLE. — What  current  will  be  taken  by  the  motor  of  Fig.  60,  it 
being  known  that  it  is  delivering  32  h.p.  at  its  pulley,  P^that  its  efficiency 
at  this  load  is  88  per  cent.,  and  that  the  e.m.f.  impressed  across  its  ter- 
minals as  read  by  the  voltmeter  is  220  volts.  SOLUTION. — Substitute 
in  equation  (16):  7  =  (h.p.0  X  74,600)  -T-  (E  X  E)  =  (32  X  74,600)  •*- 
(88  X  220)  =  2,387,200  -T-  19,360  =  123.3  amp.  That  is,  an  ammeter, 
if  inserted  at  7,  would  read  123.3  amp. 
4 


SECTION  2 
MANAGEMENT  OF  DIRECT-CURRENT  GENERATORS 

73.  To  Start  a  Shunt-wound  Generator.* — Note  the  di- 
rections in  76  concerning  the  oiling  arrangements  and  bringing 
the  machine  up  to  speed.     (1)  See  that  the  machine  is  entirely 
disconnected  from  the  external  circuit.     This  is  not  always 
necessary,  but  is  safest.     See  that  the  field  resistance  is  all  in 
circuit.     (2)  Start  the  armature  turning.     (3)  When  the  arma- 
ture has  attained  speed,  cut  out  field  resistance  until  the  vol- 
tage of  the  machine  is  normal  or  equal  to  that  on  the  bus-bars. 
(4)  Close  the  line  switch,  watching  the  ammeter  and  volt- 
meter and  make  further  adjustment  with  the  field  rheostat  if 
necessary. 

74.  To  Shut  Down  a  Shunt-wound  Generator. — (1)  Reduce 
the  load  insofar  as  possible  by  inserting  resistance  in  the  shunt- 
field  circuit  with  the  field  rheostat.     (2)  Throw  off  the  load 
by  opening  the  circuit-breaker,  if  one  is  used,  otherwise  open 
the  feeder  switches  and  finally  the  main  generator  switches. 
(3)  Shut  down  the  driving  machine.     (4)  Wipe  off  all  oil  and 
dirt,  clean  the  machine  and  put  it  in  good  order  for  the  next 
run. 

76.  Parallel  Operation  of  Shunt  Generators. — As  suggested 
in  Art.  14  shunt-wound  generators  do  not  operate  successfully 
in  parallel  because  they  do  not  divide  the  load  well  and  the  volt- 
age of  one  is  liable  to  rise  above  that  of  another  and  drive  it  as  a 
motor.  When  it  is  running  as  a  motor  its  direction  of  rotation 
will  be  the  same  as  when  it  was  generating,  hence,  the  oper- 
ator must  watch  the  ammeters  closely  for  an  indication  of 
this  trouble.  Shunt  generators  are  now  seldom  installed  and 
are  seldom  operated  in  parallel,  although  they  can  sometimes 

*  Westinghouse  Elec.  &  Manfg.  Co. 

50 


SEC.  2] 


DIRECT-CURRENT  GENERATORS 


51 


forPaneh 


be  made  to  work  that  way.  Where  there  are  several  in  a 
plant  the  best  arrangement  is  to  divide  the  total  load  between 
them,  giving  each  its  own  distinct  circuit.  Fig.  62  shows  the 
connections  for  shunt  generators  that  are  to  be  operated  in 
parallel. 

76.  To  Start  a  Compound- 
wound  Generator. — (1)  See 
that  there  is  enough  oil  in  the  f , 
bearings,  that  the  oil  rings  are 
working,  and  that  all  field  re- 
sistance is  cut  in.  (2)  Start 
the  prime  mover  slowly  and 
permit  it  to  attain  normal 
speed.  See  that  the  oil  rings 
are  working.  (3)  When  ma- 
chine is  rotating  at  normal 
speed,  cut  out  field  resistance 


Equalizer 
^     Buy 


^Positive  (+)  Bus-Bar. 


FIG.  61. — Equalizer  carried  di- 
rectly between  machines. 


until  voltage  of  the  machine  is  normal,  that  is,  equal  to  or  a 
trifle  above  that  on  the  bus-bars.  (4)  Throw  on  the  load. 
If  three  separate  switches  are  used,  as  in  Figs.  13  and  61, 
close  the  equalizer  switch  first,  the  series-coil  line  switch 


, — & 


FIG.  62. — Connections  for  shunt  generators  for  parallel  operation. 

second,  and  the  other  line  switch  third.  If  a  three-pole  switch 
is  used,  as  in  Figs.  22,  63,  64,  and  65,  all  three  poles  must  of 
course,  be  closed  at  the  same  time.  (5)  Watch  the  voltmeter 
and  ammeter  and  adjust  the  field  rheostat  until  the  machine 
takes  its  share  of  the  load.  A  machine  generating  the  higher 


52 


ELECTRICAL  MACHINERY 


[ART.  77 


voltage  will  take  more  than  its  share  of  the  load  and  if  its 

voltage  is  too  high  it  will  run 
the  other  as  a  motor. 

77.  To  Shut  Down  a  Com- 
pound-wound  Generator 
Operating  in  Parallel  with 
Others. — (1)  Reduce  the  load 
as  much  as  possible  by  throw- 
ing in  resistance  with  the  field 
rheostat.  (2)  Throw  off  the 
load  by  opening  the  circuit- 
breaker,  if  one  is  used,  other- 
wise open  the  main  generator 
switches.  (3)  Shut  down  the 
driving  machine.  (4)  Wipe 


7-'FieIofs—~ 
FIG.  63. — Connections  for  one 
unit  of  two-generators  in  series  serv- 
ing a  three-wire  system.  (This  out- 
fit is  not  arranged  for  parallel  opera- 
tion.) 


$  in.  Breaker 

Ground 
Detector 
Lamps 


off  all  oil  and  dirt,  clean  the 
machine  and  put  it  in  good 
order  for  the  next  run.  If 
the  machine  is  operating  in- 
dependently and  no  motors 

are  connected  to  the  circuit,  close  the  engine  throttle  valve 

and    permit    the    engine  . _._  . 

and  generator  to  come  to 

rest.     Turn  all  resistance 

in     the     field     rheostat. 

Open    the    main    switch. 

Where  motors  are  served 

they  must  be  disconnect- 
ed first.  If  they  are  not, 

a  loaded  motor  may  stop 

when  the  impressed  volt- 
age decreases  somewhat 

below  normal.     Then, 

since  its  armature  is  not 


•Shunt  Field 


'Shunt Fiel(i_ 


FIG.  64. — Diagram  of  connections  of 
turning,  it  IS  in  effect  a  two  direct-current  commutating-pole 
short-circuit  and  may  ge.nerators  in  parallel  with  one  generator 
.  .  .  ,  '  without  commutatmg  poles, 

blow  fuses  or  make  other 

trouble. 


SEC.  2] 


DIRECT-CURRENT  GENERATORS 


53 


78.  In  Starting  and  Shutting  Down  Three-wire  Generators, 

and  two-wire  machines  serving  a  three-wire  system  (diagrams 
for  the  parallel  operation  of  which  are  shown  in  Figs.  66,  63, 


Post  five  Bus  • 


Neutral  Bus  -. 


Negative  Bus-*. 


•Positive   '--Negative 
tqua/izer     Equalizer 
Bus  Bui        c 


•Series  Field 


Series  Held, 


FIG.  65. — Arrangement  of  two  groups  of  two-wire  generators  operating 
in  parallel  feeding  a  three-wire  system. 


I-  Two-Wire  Generator 
Connected  1o  Bus  Bows 


-  Two, Two-Wire  Generators  Connected  to  Bus  Bars 
For  Independent  Operation  on  a  Three- Wire  System 


FIG.  66.— Connections  for  two-wire  generators. 

44  and  67)  the  same  general  procedure  is  followed  as  with 
ordinary  two-wire  units  as  above  described. 

79.  Parallel  Operation  of  Compound-wound  Generators*  is 
readily  effected  if  the  machines  are  of  the  same  make  and 

•  Westinghouse  Eleo.  &  Manfg.  Co. 


54 


ELECTRICAL  MACHINERY 


[ART.  80 


voltage  or  are  designed  with  similar  electrical  characteristics. 
The  only  change  usually  required  is  the  addition  of  an  equalizer 
connection  between  machines.  If  the  generators  have  different 


•Series  Fields' 


FIG.  67. — Arrangement  of  connections  for  the  parallel  operation  on  a 
three-wire  system  of  a  three-wire  generator  and  two,  two-wire  generators 


in  series. 


compounding  ratios  it  may  be  necessary  to  readjust  the  series- 
field  shunts  to  obtain  uniform  conditions. 

80.  Testing  for  Polarity. — When    a    machine    that   is  to 
operate  in  parallel  with  others  is  connected  to  the  bus-bars 


MainSwfrh.        OD^fafcJwWr 


* 


I  10  Volt 
Jncandescent 
Lamp 


—Armature      Test  with  Lamps. 
I  Correct  Polarities- 


r;j  Voltmeter' 

T«t  with  Voltmeter 


FIG.  68.— Tests  for  polarity. 

for  the  first  time  it   should   be   tested   for   polarity.     The 
-f  lead  of  the  machine  should   connect  to  the  +  bus-bar 


SEC.  2]  DIRECT-CURRENT  GENERATORS  55 

and  the  —  lead  to  the  —  bus-bar  (Fig.  68,  /).  The  machine 
to  be  tested  should  be  brought  up  to  normal  voltage,  but 
not  connected  to  the  bars.  The  test  can  be  made  with  two 
lamps  (Fig.  68,  II),  each  lamp  of  the  voltage  of  the  circuit. 
Thus,  each  is  temporarily  connected  between  a  machine 
terminal  and  bus  terminal  of  the  main  switch.  If  the  lamps 
do  not  burn,  the  polarity  of  the  new  machine  is  correct, 
but  if  they  burn  brightly  its  polarity  is  incorrect  and  should 
be  reversed.  A  voltmeter  can  be  used  (Fig.  68,  III).  A 
temporary  connection  is  made  across  one  pair  of  outside  termi- 
nals and  the  voltmeter  is  connected  across  the  other  pair. 
No  deflection  or  a  small  deflection  indicates  correct  polarity. 
(Test  with  voltmeter  leads  one  way  and  then  reverse  them,  as 
indicated  by  the  dotted  lines.)  A  full-scale  deflection  indi- 
cates incorrect  polarity.  Use  a  volt-meter  having  a  voltage 
range  equal  to  twice  the  voltage  on  the  bus-bars. 

81.  An  Equalizer,  or  Equalizer  Connection,  connects  two 
or  more  generators  operating  in  parallel  at  a  point  where  the 
armature  and  series-field  leads  join  (see  Fig.  13),  thus  con- 
necting the  armatures  in  multiple  and  the  series  coils  in  mul- 
tiple, in  order  that  the  load  will  divide  between  the  genera- 
tors in  proportion  to  their  capacities.  The  arrangement  of 
connections  to  a  switchboard*  is  shown  in  Fig.  22.  Con- 
sider, for  example,  two  compound-wound  machines  operating 
in  parallel  without  an  equalizer.  If,  for  some  reason,  there 
is  a  slight  increase  in  the  speed  of  one  machine,  it  would 
take  more  than  its  share  of  load.  The  increased  current 
flowing  through  its  series  field  would  strengthen  the  magnet- 
ism, raise  the  voltage,  and  cause  the  machine  to  carry  a 
still  greater  amount  until  it  carried  the  entire  load.  Where 
equalizers  are  used,  the  current  flowing  through  each  series  coil 
is  proportional  to  the  resistance  of  the  series-coil  circuit  and  is 
independent  of  the  load  on  any  machine;  consequently  an 
increase  of  voltage  on  one  machine  builds  up  the  voltage  of 
the  others  at  the  same  time,  so  that  the  first  machine  cannot 
take  all  the  load  but  will  continue  to  share  it  in  proper 
proportion  with  the  other  generators. 

*  Westinghouse  Eleo.  &  Manfg.  Co. 


56  ELECTRICAL  MACHINERY  [ART.  82 

82.  Connecting  Leads  for  Compound  Generators. — Be  cer- 
tain that  all  the  cables  which  connect  from  the  various  ma- 
chines to  the  bus-bars  are  of  equal  resistance.     This  means 
that    if  the   machines   are    at  different  distances  from  the 
switchboard,  different  sizes  of  wire  should  be  used,  or  resist- 
ance inserted  in  the  low-resistance  leads;  see  Art.  84.     With 
generators  of  small  capacity  the  equalizer  is  usually  carried 
to  the  switchboard,  as  suggested  in  Fig.  64,  but  with  larger 
ones   it   is   carried   under   the    floor   directly   between   the 
machines   (Fig.  61).     In  some  installations  the  positive  and 
the  equalizer  switch  of  each  machine  are  mounted  side  by  side 
on  a  pedestal  near  the  generator  (Fig.  61).     The  difference  in 
potential  between  the  two  switches  is  only  that  due  to  the 
small  drop  in  the  series  coil.     The  positive  bus-bar  is  carried 
under  the  floor  near  the  machines.     This   permits  of   leads 
of  minimum  length.     Leads  of  equal  lengths  should  be  used 
for  generators  of  equal  capacities.     If  the  capacities  are  un- 
equal  (see  Art.  84)  it  may  be  necessary  to  loop  the  leads. 
See  Fig.  61. 

83.  Ammeters    for   Compound   Generators   should,  as  in 
Fig.  22,  always  be  inserted  in  the  lead  not  containing  the 
compound  winding.     If  cut  in  the  compound-winding  lead, 
the  current   indications  will   be  inaccurate  because  current 
from  this  side  of  the  machine  can  flow  either  through  the 
equalizer  or  the  compound-winding  lead. 

84.  To  Adjust  the  Division  of  Load  Between  Two  Com- 
pound-wound Generators. — First  adjust  the  series  shunts  of 
both  machines  so  that,  as  nearly  as  possible,  the  voltages  of 
both  will  be  the  same  at  K,   M,  %,  and  full-load.     Then 
connect  the  machines  in  parallel,  as  suggested  in  Fig.  13,  for 
trial.     If  upon  loading,  one  machine  takes    more    than    its 
share  of  the  load  (amperes),  increase  the  resistance  of  the 
circuit  through  its  series-field-coil  path  until  the  load  divides 
between  the  machines  in  proportion  to  their  capacities.    Only 
a  small  increase  in   resistance   is   usually   necessary.     The 
increase  may  be  provided  by  inserting  a  longer  conductor 
between  the  generator  and  the  bus-bar,  or  iron  or  German- 
silver  washers  can  be  inserted  under  a  connection  lug.     In- 


SEC.  2] 


DIRECT-CURRENT  GENERATORS 


57 


asmuch  as  (when  machines  are  connected  in  parallel)  ad- 
justment of  the  series-coil  shunt  affects  both  machines 
similarly,  the  division  of  load  between  the  two  machines 
cannot  be  altered  by  making  such  adjustment. 

85.  Operation  of  a  Shunt  and  a  Compound  Dynamo  in 
Parallel  is  not   successful   because  the   compound   machine 
will  take  more  than  its  share  of  the  load  unless  the  shunt 
machine  field  rheostat  is  adjusted  at  each  change  in  load. 

86.  Three  -wire   Direct  -current  Generators    Can  be  Op- 
erated in  Multiple*  with  each  other  and  in  multiple  with  other 
machines  on  the  three-  wire  system.     See  Figs.  40,  41,  42, 
43,  44,  45  and  67.     When  operating   a  three-wire,  250-volt 


V—  ...........................  Generating  Station-  ...........  —>k  -----  .Line  .............  4«—  -  Load-  ------- 


FIG.  69. — Two  125-volt,  two-wire  generators  and  one  250-volt  three- 
wire  generator  connected  for  parallel  operation. 

generator  in  multiple  with  two  two- wire,  125-volt  generators 
(Figs.  40,  67  and  69)  the  series  field  of  the  two  two-wire 
generators  must  be  connected,  one  in  the  positive  side  and 
one  in  the  negative  side  of  the  system,  and  an  equalizer  must 
be  run  to  each  machine.  Similarly,  when  operating  a  three- 
wire,  250-volt  generator  in  multiple  with  a  250-volt,  two- 
wire  generator  (Figs.  41  and  42)  the  series  field  of  the  250- 
volt,  two-wire  generator  must  be  divided  and  one-half 
connected  to  each  outside  wire.  The  method  of  doing  this 
is  to  disconnect  the  connectors  between  the  series-field  coils 
and  reconnect  these  coils  so  that  all  the  north-pole  fields 

*  WEBTINQHOUBE  PUBLICATION. 


58  ELECTRICAL  MACHINERY  [ART.  87 

will  be  in  series  on  one  side  of  the  three-wire  system  and  all 
the  south-pole  fields  in  series  on  the  other  side  of  the  system. 

87.  The    Connections   Where   Two -wire    Generators  are 
Operated  to  Feed  a  Three-wire  System  are,  where  the  units 
are  operated  in  parallel,  shown  diagrammatically  in  Fig.  65. 
It  will  be  noted  that  both  a  positive  and  a  negative  equalizer 
bus  are  required.     Fig.  63  illustrates  the  connections  where 
only  one  pair  of  two-wire  generators  is  utilized  to  serve  a 
three-wire  system   and  a  similar   arrangement  is  shown  in 
Fig.  66,  II. 

88.  As  There   are   Two    Series    Fields,    Two    Equalizer 
Buses  are  Required  When  Several  Three-wire   Machines 
are  Installed  (see  Figs.  40  and  41)  and  are  to  be  operated  in 
multiple.     The  two  equalizers  serve  to  distribute   the   load 
equally  between  the  machines  and  to  prevent  cross-current 
due  to  differences-  in  voltage  on  the  different  generators. 

89.  An  Ammeter  Shunt   (Si  and  $2,  Fig.  33)  is  mounted 
directly  on  each   of  the  terminal  boards  of  the  three-wire 
machines.     The  total   current    output   of  the  machine   can 
thereby  be  read  at  the  switchboard.     As  the  shunts  are  at  the 
machine,  there  is  no  possibility  for  current  to  leak  between 
generator   switchboard  leads    without   indicating  a   reading 
on  the  ammeters.     Two   ammeters   must   be   provided   for 
reading  the  current  in  the  outside  wires.     It  is  important 
that  the  current  be  measured  on  both  sides  of  the  system, 
for  with  an  ammeter  in  one  side  of  the  system  only,  it  is 
possible  for  a  large  unmeasured  current  to  flow  in  the  other 
side  with  disastrous  results. 

90.  Switchboard  Connections  for  Three -wire  Generators 
are  shown  in  principle  in  preceding  illustrations.     Fig.  43  is 
a  diagrammatic  representation  of  the   switchboard   connec- 
tions  for  two   three-wire  generators  operated  in  multiple.* 
Two  ammeters  indicate  the  unbalanced  load.     The  positive 
lead  and  equalizer  are  controlled  by  a  double-pole  circuit- 
breaker;    the    negative  lead  and  equalizer  likewise.     Note 
that  both  the  positive   and  negative   equalizer   connections 
as  well  as  both  the  positive  and  negative  leads  are  run  to  the 

•WESTINGHOTJSE  PUBLICATION. 


SEC.  2]  DIRECT-CURRENT  GENERATORS  59 

circuit-breakers  in  addition  to  the  main  switches  on  the 
switchboard.  It  is  necessary  that  this  be  done  in  all  cases. 
Otherwise,  when  two  or  more  machines  are  running  in  mul- 
tiple and  the  breaker  comes  out,  opening  the  main  circuit 
to  one  of  them  but  not  breaking  its  equalizer  leads,  its  am- 
meter is  left  connected  to  the  equalizer  bus-bars  and  current 
is  fed  into  it  from  the  other  machines  through  the  equalizer 
leads,  either  driving  it  as  a  motor  or  destroying  the  arma- 
ture winding.  See  also  Figs.  40  and  42. 

91.  Commutating-pole  Machines  Will   Run    in   Multiple 
with  each  other  and  with  non-commutating  pole  machines 
provided  correct  connections    are    made.     See   illustrations. 

The  series-field  windings  on  commutating-pole  machines 
are  usually  less  powerful  than  on  non-commutating-pole;  and 
particular  attention  should,  therefore,  be  paid  to  insuring 
the  proper  drop  in  accordance  with  instructions  of  Art.  84.  A 
connection  diagram  is  shown  in  Fig.  64. 

92.  How  to  Reverse  the  Direction  of  Rotation  of  Direct- 
current  Generators  and  Motors  for  a  shunt-wound  machine 
is  indicated  in  Fig.  70.     Rotation  is  clockwise  when,  facing  the 


I  Clockwi'.p  Rntntmn  ^  Counterwise  Rotation.  M  Counterwise  Rotation 

l-ation.  (i/ne  Wire  p0ianfy  Reversed)  (Armafure  Leads  fatnM/J 

FIG.  70. —Changing  rotation  direction  of  shunt  machine. 

commutator  end  of  a  machine,  the  rotation  is  in  the  direction 
of  the  hands  of  a  clock.  Counter-clockwise  rotation  is  the 
reverse.  It  is  desirable,  when  changing  the  direction  of  rota- 
tion, not  to  reverse  the  direction  of  current  through  the  field 
windings.  If  it  is  reversed  the  magnetism  developed  by  the 
windings  on  starting  will  oppose  the  residual  magnetism  and 
the  machine  may  not  "build-up."  Connections  for  reversing 
compound  machines  are  shown  in  Fig.  71.  A  multipolar  ma- 
chine can  be  reversed  as  shown  by  reversing  the  brushes  on 
the  studs  and  then  relocating  them  on  the  neutral  points. 


60 


ELECTRICAL  MACHINERY 


[ART.  93 


93.  To  Reverse  the  Direction  of  Rotation  of  a  Commutating- 
pole  Generator  (Fig.  25)  reverse  the  shunt  and,  if  there  are 
such,  the  series  fields,  as  in  an  ordinary  generator.     See  Figs. 
70  and  71. 

94.  When  Starting  Up,  a  Generator  May  Fail  to  Excite  It- 
self. * — This  may  occur  even  when  the  generator  operated  per- 
fectly during  the  preceding  run.     Usually  this  trouble  is  caused 
by  a  loose  connection  or  break  in  the  field  circuit,  by  poor 
contact  at  the  brushes  due  to  a  dirty  commutator  or  perhaps 
to  a  fault  in  the  starting  box  or  rheostat,  or  incorrect  position 
of  brushes.     Examine  all  connections;  try  a  temporarily  in- 
creased pressure  on  the  brushes:  look  for  a  broken  or  burnt- 


Series Field*. 


I.  Clockwise  Rotation. 


H  Counterclockwise  Rotation. 
(Main  Leads  Reversed} 


TY.  Y 

Multipolar  Machine 
Clockwise  Kotation.  Counterclackwise  Rotation- 


IE.  Counterwise  Retortion. 
(Armature  Leadi  Reversed) 

FIG.  71.  —  Changing  rotation  of  compound  machine. 


out  resistance  coil  in  the  rheostat.  An  open  circuit  in  the 
field  winding  may  sometimes  be  traced  with  the  aid  of  a 
magneto  bell;  but  this  is  not  an  infallible  test  as  some  mag- 
netos will  not  ring  through  a  circuit  of  such  high  resistance 
and  reactance  even  though  it  be  intact.  If  no  open  circuit  is 
found  in  the  starting  box  or  in  the  field  winding,  the  trouble 
is  probably  in  the  armature.  But  if  it  be  found  that  nothing 
is  wrong  with  the  connections  or  the  winding  it  may  be 
necessary  to  excite  the  field  from  another  generator  or  some 
other  outside  source  as  described  below. 

94a.  To  Excite  a  Field  from  an  Outside  Source.  —  Calling 
the  generator  it  is  desired  to  excite  No.  1,  and  the  other  ma- 

*  WESTINQHOUSSS  INSTRUCTION  BOOK. 


SEC.  2] 


DIRECT-CURRENT  GENERATORS 


61 


-  Bus  Bar 


Equalizer 

1+  Bus  Bar 

M~&               C« 

. 

.  '[-* 

'Armature 

FIG.    72. — Exciting    a    generator 
with  an  external  source  of  e.m.f. 


chine  from  which  current  is  to  be  taken  No.  2,  the  follow- 
ing procedure  should  be  followed.  Open  all  switches  and 
remove  all  brushes  from  generator  No.  1 ;  connect  the  positive 
brush  holder  of  generator  No.  1  with  the  positive  brush  holder 
of  generator  No.  2;  also  connect  the  negative  holders  of  the 
machines  together  (it  is  desirable  to  complete  the  circuit 
through  a  switch  having  a  fuse 
of  about  5  amp.  capacity  in 
series).  Close  the  switch. 
Where  the  generator  in  trouble 
is  connected  to  bus-bars  fed  by 
other  generators,  the  same  re- 
sult can  be  effected  by  insulat- 
ing the  brushes  of  the  machine 
in  trouble  from  their  commu- 
tator and  closing  the  main 
switch.  See  Fig.  72.  If  the 
shunt  winding  of  generator  No.  1  is  intact  its  field  will  show 
considerable  magnetism.  If  possible,  reduce  the  voltage  of 
generator  No.  2  before  opening  the  exciting  circuit;  then 
break  the  connections.  If  this  cannot  be  done,  throw  in  all 
the  rheostat  resistance  of  generator  No.  1;  then  open  the 
switch  very  slowly,  lengthening  out  the 

Shunt FMd  arc  wnich  will  be  formed  until  it  breaks. 

95.  A  Simple  Means  for  Getting  a 
Compound-wound  Machine  to  Pick  Up 
is   to   short-circuit  it  through   a  fuse 
having  approximately  the  current  capa- 
FIG.  73. — One  method  city  of  the  generator.      See  Fig.   73. 
T  If  sufficient  current  to  melt  this  fuse 
up.  is  not  generated,  it  is  evident  that  there 

is  something  wrong  with  the  armature, 

either  a  short-circuit  or  an  open  circuit.  If,  however,  the  fuse 
has  blown,  make  one  more  attempt  to  get  the  machine  to 
excite  itself.  If  it  does  not  pick  up,  it  is  evident  that  some- 
thing is  wrong  with  the  shunt  winding  or  connections. 

96.  If  a  New  Machine  Refuses  to  Excite  and  the  connections 
seem  to  be  all  right,  reverse  the  connections,  i.e.,  connect  the 


Armcrtvr<s 


62 


ELECTRICAL  MACHINERY 


[ART.  97 


frame— 


^-Copper* 
Strap 


wire  which  normally  leads  from  the  positive  brush,  to  the  nega- 
tive brush  and  the  wire  which  normally  leads  from  the 
negative  brush,  to  the  positive  brush.  If  this  change  of  con- 
nections does  not  correct  the  difficulty,  change  the  connections 

back  as  they  were  and  locate 
the  fault  as  previously  sug- 
gested. 

97.  Sometimes  When  a 
Generator  Fails  to  Excite, 
tapping  the  iron  of  the  field 
structure  with  a  hammer  will 
correct  the  difficulty. 
Another  method  that  is  often 
successful  is  shown  in  Fig.  74;" 
flat  copper  strips  about  Y±  in. 


FIG.  74. — Metal-strip  brushes  in 
machine  which  will  not  excite. 


wide  are  held  in  position  by 
an  assistant  across  the  brush- 
holder  studs  so  that  the  ends 
of  the  strips  will  contact  with  the  commutator  on  the  line  AiA2. 
These  strips  form  a  path  to  the  armature  which  is  of  much 
lower  resistance  than  the  path  through  the  brushes.  There- 
fore, when  high  brush-contact  resistance  is  the  difficulty,  this 
expedient  is  very  effective. 


•Frame 


Frame 


-Pole 
Piece 


"Connecting 
Lead  I 

I r on  Bar S'* 
or  a  Couple 
\        of  Nail;,     /— 


I-Proper  Polarities 


*  i  i\jy^<    rvivnni^j  B- Testing  Polarities 

FIG.  75. — Proper  polarity  sequence  for  the  poles  of  a  direct-current 
machine  and  one  method  of  testing  for  polarity. 

98.  Polarity  of  Field  Can  Be  Tested  in  Two  Ways.*  (The 
other  "nail"  method  is  described  in  detail  in  following  Art. 
100: — First,  by  using  a  compass,  bringing  it  near  the  various 


Raymond's  MOTOR  TROUBLES  and  POWER,  July  21,  1914,  p.  86. 


SEC.  2} 


DIRECT-CURRENT  GENERATORS 


63 


poles  and  noting  the  direction  of  the  deflection  of  the  needle. 
Since  in  all  direct-current  generators  and  motors  the  poles 
should  alternate  in  magnetic  polarity,  Figs.  75  and  76,  I  (in 
one  pole  the  magnetism  " coming  out"  and  the  next  "going 


M- 

o/o  not 
Alternate  in 
Direction 


Connector         Alternate  in  H~  Incorrect 

FIG.  76. — Showing  correct  and  incorrect  polarities  of  field  coils. 


in")  it  follows  that  a  certain  end  of  a  compass  needle  will 

point  toward  one  pole  and  away  from  the  next  when    con- 

ditions  are  normal.     If,   however,  two  adjacent  poles  show 

similar  magnetism,  the  trouble  is  located,  and  the  offending 

spool  should  be  reversed.     This  may  have  to  be  done  by  rota- 

ting the  field  coil,  on  its  axis, 

through  180°  and  then  recon- 

necting    it.     The     mechanical 

construction  of  some  coils  will 

not  permit  their  being  turned 

end-for-end. 

99.  Direction  of  Magnetism 


'Direction 
ofCurrent 


•  « 

Lines  of  Force 


FlG  77._Current  and  magneti- 
is  Determined  by  the  Follow-  zation  of  pole. 
ing    Rule.  —  "Looking    at    the 

face  of  an  electromagnet  (such  as  the  field  spool  of  a  motor), 
a  pole  will  be  north  if  the  current  is  flowing  around  it  in  a 
direction  opposite  to  the  motion  of  the  hands  of  a  watch,"  Fig. 
77,  and  south  if  in  the  same  direction  as  the  motion  of  the 
hands  of  a  watch. 


64 


ELECTRICAL  MACHINERY 


[ART.  100 


100.  Another  Method  of  Determining  Whether  the  Direc- 
tion of  Magnetism  of  the  Poles  is  Correct  is  to  use  two  or- 
dinary nails,  their  lengths  depending  upon  the  distance  be- 
tween pole-tips.  Instead  of  using  two  nails,  a  piece  of  iron 
rod  long  enough  to  reach  from  one  pole  to  another — A  or 
B,  Fig.  75,  // — may  be  employed.  The  point  of  one  nail 
should  touch  one  pole-tip,  the  point  of  the  other  nail  the 
other  pole-tip,  and  the  heads  of  the  nails  should  touch  each 
other.  When  the  current  flows  around  the  field  spools, 
the  polarity  between  any  two  poles  is  properly  related  if 
the  nails  placed  as  suggested  stick  together  by  the  magnet- 
ism or  if  the  iron  bar  is  held  strongly  between  the  poles.  If 
there  is  little  or  no  tendency  for  the  nails  or  the  bar  to  be 


Glued  in     5-C-C.  Copper  Wire 
Place 


//I    ^PinHoMn0\*.  --Wooc/en  Handle 

*+      Iron  Core       B\ 


•Terminal  Lead 


FIG.  78. — A  polarity-testing   coil  used  by  the   British   Westinghouse 

Company. 

held  to  the  poles  the  polarity  of  the  two  adjacent  poles  is  the 
same  and  therefore  wrong. 

101.  An  Inductive  Polarity  Tester*  is  shown  in  Fig.  78. 
In  use  the  leads  A  and  B  of  the  coil  are  connected  to  a  direct- 
current  millivoltmeter.  Then,  if  the  coil  is  moved  toward 
an  excited  field  magnet,  the  millivoltmeter  needle  will  indicate 
a  momentary  deflection  in  one  direction.  This  direction 
will  change  with  the  polarity  of  the  field  coil  under  test. 
Usually  a  greater  deflection  will  be  obtained  if  the  testing  coil 
is  moved  slowly  toward  the  excited  field  and  then  quickly 
withdrawn.  If  it  is  withdrawn  from  a  south  pole  it  will 
move,  say  to  the  right.  Then  if  it  is  withdrawn  from  a  north 
pole  it  will  move  toward  the  left.  Thus,  by  testing  the  poles 

*  ELECTRICAL  WORLD,  Jan.  9,  1915,  p.  102. 


SEC.  2] 


DIRECT-CURRENT  GENERATORS 


65 


around  a  direct-current  machine  in  rotation  it  is  possible  to 
ascertain  whether  the  connections  have  been  made  correctly 
or  incorrectly.  By  exercising  caution  this  device  can  be  used 
with  machines  which  are  in  operation.  Where  a  milli- 
voltmeter  is  not  available  a  moving-coil  ammeter,  which  has 
had  its  shunt  disconnected,  may  be  utilized.  Low-reading  volt- 
meters (1  or  3  volts)  can  be  employed  provided  the  test- 
ing coil  is  wound  with  many  turns  pf  small-diameter  wire. 

102.  In  Placing  the  Field  Coils  on  a  Direct-current  Genera- 
tor or  Motor  they  should  be  tested,  prior  to  mounting  on  the 
machine,  to  insure  that  their  polarities  will  alternate  as  shown 
in  Fig.  76,  7,  around  the  frame.  Hence,  before  installation, 
the  coils  should  be  arranged  on  the  floor  and  connected  in  series 
across  a  source  of  voltage  equal  to  that  which  will  be  im- 


•Direction  of  Current 

— >      '  ---> 


Arc  in  which  nail  should  be  Moved 


Magnetiieai  Nail-' 


FIG.  79.  —  Determination  of  polarities  of  field  coils  with  a  magnetized 
nail.     (POWER,  July  21,  1914,  p.  86). 

pressed  on  the  group  of  field  coils  when  they  are  in  operation 
on  the  machine.  Then,  the  normal  exciting  current  should 
be  permitted  to  flow  through  the  coils,  and  a  compass  (Fig. 
76,  7)  should  be  held  in  the  core  space  of  each  of  the  coils 
in  succession.  If  the  coils  are  correctly  arranged,  the  needle 
will  point  alternately  north  and  south  in  adjacent  coils  as 
shown.  But,  if  the  coils  are  not  properly  connected  the  com- 
pass needle  will  point  in  the  same  direction  in  adjacent  coils,  as 
shown  at  A  and  B  in  77.  Where  a  coil  indicates  incorrect 
polarity  it  should  be  turned  end  for  end.  That  is,  in  Fig.  76, 
II,  coil  B  should  be  turned  end  for  end.  If  no  compass  needle 
is  available,  a  nail,  or  preferably  a  short  piece  of  hard-steel 
wire,  may  be  magnetized  and  used  for  a  polarity  indicator 
as  illustrated  in  Fig.  79.  Such  a  piece  of  metal  can  be  made 


66  ELECTRICAL  MACHINERY  [ART.  103 

into  a  permanent  magnet  by  holding  it  in  the  core  space  of 
one  of  the  coils.  When  the  coils  are  properly  arranged,  the 
nail  or  piece  of  steel  wire  can  be  moved  from  one  coil  to  the 
other  along  the  curves  such  as  those  indicated  by  the  dotted 
lines  in  the  illustration.  In  making  this  test  with  a  nail  it  is 
necessary  to  exercise  precaution  to  insure  that  the  polarity  of 
the  nail  or  wire  does  not  become  reversed  while  the  test  is 
being  conducted. 

103.  In  Determining  the  Polarities  of  Commutating-pole 
Windings.* — If  the  armature  of  a  six-pole  generator  is  rotating 
in  a  clockwise  direction,  then  the  commutating  pole  which 
stands  in  the  position  corresponding  to  the  11  o'clock  mark 
of  the  clock  should  have  the  same  polarity  as  the  main  pole 
corresponding  to  the  12  o'clock  mark  and  so  on  all  the  way 
round.     On  the  other  hand,  each  commutating  pole  of  a  motor 
should  have  the  same  polarity  as  the  main  pole  preceding  it 
in  the  direction  of  rotation.     With  a  six-pole  motor  running  in 
a  clockwise  direction,  the  commutating  pole  in  the  11  o'clock 
position  should  have  the  same  polarity  as  the  main  pole  in 
the  10  o'clock  position. 

103a.  Sometimes  the  Commutating  Poles  of  a  Machine 
Show  Correct  Polarity  When  There  is  a  Good  Load  on  the 
Machine  but  Behave  Irregularly  on  Light  Loads. — This  may 
be  due  to  slight  irregularities  in  the  mechanical  construction  of 
the  machines.  If  a  commutating  pole  is  not  in  the  center 
of  its  neighboring  poles,  but  is  nearer,  say,  to  the  main  north 
pole  than  to  the  main  south  pole,  then  the  commutating-pole 
tip  will  show  south  polarity  even  if  there  is  no  current  flowing 
in  its  windings.  Therefore,  with  a  very  light  load,  the  current 
in  a  commutating-pole  winding,  and  tending  to  make  the  pole, 
say,  a  north  pole  may  not  be  strong  enough  to  overcome  the 
polarity  caused  by  the  unequal  setting.  The  same  condition 
will  occur  if  the  air  gap  between  the  armature  and  north  pole 
is  smaller  than  that  between  the  armature  and  south  pole. 

104.  The  Management  of  Brushes  on  Direct-current  Gen- 
erators is  treated  under  the  general  heading  of  brush  troubles 
in  Art.  190  and  succeeding  articles. 

*  ELECTRICAL  WORLD,  Jan.  9,  1915,  p.  102. 


SEC.  2] 


DIRECT-CURRENT  GENERATORS 


67 


105.  In  Caring  for  Commutators  they  should  be  kept  smooth 
by  the  occasional  use  of  No.  00  sandpaper.  A  small  quantity  of 
high-grade,  light-body  oil  may  be  used  as  a  lubricant.  The 
lubricant  should  be  applied  to  high-voltage  generators  on  a 
piece  of  cloth  attached  to  the  end  of  a  dry  stick.  If  the  com- 
mutator becomes  "out  of  true"  it  should  be  turned  down. 
By  using  a  special  slide  rest  and  tool  this  can  be  done  while 
running  the  engine  at  a  reduced  speed  without  removing  the 
rotating  part  from  the  bearings.  Inspect  the  commutator 


.,  Lifting  Loops 


Armature 
Winding  — 


"'Commutator 
I- How  Spreader  is  Used 

k- -Bolts  to  Prevent  Splitting 


FIG.  80. — The  second  position  in 
removing  an  armature. 


IF,' 

Groove  for  Sting-  -  • ' 
l-Plcm  View  of  Spreader 

FIG.  81. — Showing  the  applica- 
tion of  a  spreader  to  prevent  ab- 
rasion of  the  armature  windings. 


surface  carefully  to  see  that  the  copper  has  not  been  turned 
over  from  segment  to  segment  in  the  mica  and  remove  by  a 
scraper  any  particles  of  copper  which  may  be  found  embedded 
in  the  mica.  Keep  oil  away  from  the  mica  end-rings  of  the 
commutator  as  oily  mica  will  soon  burn  out  and  ground  the 
machine.  See  Sec.  3  for  information  relating  to  commutator 
troubles  and  their  correction. 

106.  In  Handling  an  Armature  care  must  be  exercised  to 
prevent  injury  to  its  windings.  The  armature  may  be  re- 
moved from  a  machine  with  a  rope  sling,  as  shown  in  Fig.  80. 


68 


ELECTRICAL  MACHINERY 


[ART.  107 


Crane  Hoott 


Rope  S/ing 


Commutator- 


After  it  has  been  taken  out  of  the  frame,  a  wooden  spreader 
should  be  arranged  in  the  sling  as  shown  in  Fig.  81  to  pre- 
vent the  rope  from  abrading  the  winding  of  the  commutator. 
An  armature  can  be  replaced  as  shown  in  Fig.  82.  Pieces 
of  stiff  sheet-fiber  should  be  utilized,  as  shown  in  the 
illustrations,  to  prevent  portions  of  the  frame  of  the  ma- 
chine from  damaging  the  commutator  or  winding  insulation. 

107.  Drying  Out  a  Generator  or  Motor.* — If  a  generator 
has  been  exposed  to  dampness,  before  being  started  in  regular 

service  it  should  be  operated  with 
its  armature  short-circuited  be- 
yond the  ammeters  and  with  the 
field  current  adjusted  so  as  to 
raise  temperature  to  about  70 
deg.  C.  (See  Art.  247  for  descrip- 
tion of  method  of  measuring  the 
insulation  resistance  of  a  machine; 
the  low  insulation  resistance  may 
indicate  moisture  and  vice  versa.) 
The  current  should  then  be  per- 
mitted to  flow  until  the  coils  be- 
come thoroughly  dry.  The  tem- 
perature should  not  be  allowed 
to  drop  to  that  of  the  surrounding 
FIG.  82.— The  first  position  atmosphere,  as  the  moisture 
in  removing  or  the  last  position  would  then  again  be  condensed 
in  replacing  an  armature.  , ,  .-.  -,  ,-,  ,  . 

on    the    coils,    and   the  machine 

brought  to  the  same  condition  as  at  the  start. 

108.  There  is  Always  Danger  of  Overheating  the  Wind- 
ings of  a  Machine  When  Drying  Them   with    current,   as 
the  inner  parts,   which   cannot   quickly  dissipate   the  heat 
generated   in   them    and    which    cannot   be  examined,  may 
get  dangerously  hot,  while  the  more-exposed  and  more-easily- 
cooled  portions  are  still  at  a  comparatively  low  temperature. 
The  temperature  of  the  hottest  part  accessible  should  always 
be  observed  while  the  machine  is  being  dried  out  in  this  way, 


*  WESTINGHOUSE  INSTRUCTION  BOOK. 


SEC.  2]  DIRECT-CURRENT  GENERATORS  69 

and  it  should  not  be  allowed  to  exceed  the  boiling  point  of 
water.  It  may  require  several  hours  or  even  days  to 
thoroughly  dry  out  a  machine,  especially  if  it  is  of  large 
capacity.  Large  field  coils  dry  very  slowly.  Insulation  is 
more  easily  injured  by  overheating  when  damp  than  when  dry. 


SECTION  3 

MANAGEMENT  OF  AND  STARTING  AND  CONTROLL- 
ING DEVICES  FOR  DIRECT-CURRENT  MOTORS 

109.  The  Management  of   Direct-current  Motors   is    in 
many  particulars — since  (Art.  52)   the  construction  of  these 
motors    and    generators   is   the   same — similar    to    that    of 
direct-current   generators.     It  is    suggested,    therefore,  that 
any  reader  seeking  information  on  this  subject  review  Sec.  2 
on  the  "Management  of  Direct-current  Generators." 

110.  Control  of  Direct-current  Electric  Motors.    Rheostats.  * 
— A  direct-current  motor  of  any  capacity,  when  its  armature 
is  at  rest,  offers  a  very  low  resistance  to  the  flow  of  cur- 
rent  and    an   excessive    and    perhaps    destructive    current 
would  flow  through  it  if  it  were  connected  directly  across 
the  supply  mains  while  at  rest. 

EXAMPLE. — Consider  a  motor  adapted  to  a  normal  full-load  current 
of  100  amp.  and  having  an  armature  resistance  of  0.25  ohm;  if  this 
motor  were  connected  across  a  250-volt  circuit  a  current  of  1,000  amp. 
would  flow  through  its  armature — in  other  words,  it  would  be  over- 
loaded 900  per  cent,  with  consequent  danger  to  its  windings  and  also 
to  the  driven  machine.  In  the  case  of  the  same  motor,  with  a  rheostat 
having  a  resistance  of  2.25  ohms  inserted  in  the  motor  circuit,  at  the 
time  of  starting  the  total  resistance  to  the  flow  of  current  would  be  the 
resistance  of  the  motor  (0.25  ohm)  plus  the  resistance  of  the  rheostat 
(2.25  ohms),  or  a  total  of  2.5  ohms.  Under  these  conditions  exactly 
full-load  current,  or  100  amp.,  would  flow  through  the  motor,  and 
neither  the  motor  nor  the  driven  machine  would  be  overstrained  in 
starting.  This  indicates  the  necessity  of  a  rheostat  for  limiting  the  flow 
of  current  in  starting  the  motor  from  rest. 

111.  An  Electric  Motor  is  Simply  an  Inverted  Generator; 

consequently  when  its  armature  begins  to  revolve  a  voltage 
is  generated  within  its  windings  just  as  a  voltage  is  gener- 
ated in  the  windings,  of  a  generator  when  driven  by  a 

*  The  Electric  Controller  &  Manfg.  Co. 

70 


SEC.  3] 


DIRECT-CURRENT  MOTORS 


71 


prime  mover.  This  voltage  generated  within  the  moving 
armature  of  a  motor  opposes  the  voltage  of  the  circuit 
from  which  the  motor  is  supplied,  and  hence  is  known 
as  a  "counter-electromotive  force."  The  net  voltage  tending 
to  force  current  through  the  armature  of  a  motor  when  the 
motor  is  running  is,  therefore,  the  line  voltage  minus  the 
counter-electromotive  force.  In  the  case  of  the  motor  cited 
in  the  above  example,  when  the  armature  attains  such  a 
speed  that  a  voltage  of  125  is  generated  within  its  windings, 
the  effective  voltage  will  be  250  minus  125,  or  125  volts,  and, 
therefore,  the  resistance  of  the  rheostat  may  be  reduced  to 


^hunt- 
Field 
Co/Is 


FIG.  83. — Method  of  connecting  starting  box,  cut-out  and  main  switch 
for  a  four-pole,  shunt-wound  motor. 

1  ohm  without  the  full-load  current  of  the  motor  being 
exceeded.  As  the  armature  further  increases  its  speed, 
the  resistance  of  the  rheostat  may  be  further  reduced  until, 
when  the  motor  has  almost  reached  full  speed,  all  of  the 
rheostat  resistance  may  be  cut  out,  and  the  counter-electro- 
motive force  generated  by  the  motor  will  almost  equal  the 
voltage  supplied  by  the  line  so  that  an  excessive  current 
cannot  flow  through  the  armature. 

112.  In  Practice,  a  Rheostat  (Fig.  83)  is  Provided  for 
Starting  a  Direct-current  Electric  Motor. — The  conductor 
providing  the  resistance  is  divided  into  sections  and  is  so 
arranged  that  the  entire  length  or  maximum  resistance  of 


72 


ELECTRICAL  MACHINERY 


[ART.  113 


the  rheostat  is  in  circuit  with  the  motor  at  the  instant  of 
starting  and  that  the  effective  length  of  the  conductor, 
and  hence  its  resistance,  may  be  reduced  as  the  motor 
comes  up  to  speed.  In  cutting  out  the  resistance  of  a 
starting  rheostat  it  must  not  be  cut  out  too  rapidly.  If  the 
resistance  is  cut  out  more  rapidly  than  the  armature  can 
speed  up,  a  sufficient  counter-electromotive  force  will  not  be 
generated  to  properly  oppose  the  flow  of  current,  and  the 
motor  will  be  overloaded. 

113.  Rheostatic  Controller. — If  all  the  resistance  of  the 
starting  rheostat  (see  above  paragraph)  is  not  cut  out,  the 
motor  will  operate  at  reduced  voltage,  and  hence  at  less 


Series  Mofon 


FIG.  84. — Wiring  connections  for  a  series  motor  and  its  rheostat. 

than  normal  speed.  A  rheostat  (Fig.  84)  so  arranged  that 
all  or  a  portion  of  its  resistance  may  be  left  in  a  motor  circuit 
to  secure  reduced  speeds  is  called  a  "rheostatic  controller." 
Such  rheostatic  controllers  are  used  for  controlling  series 
and  compound-wound  motors  driving  cranes  and  similar 
machinery  requiring  variable  speed  under  the  control  of  an 
operator. 

113a.  In  Starting  a  Direct-current  Motor  (see  Fig.  85),  close 
the  line  switch  and  move  the  operating  arm  of  the  rheostat 
(Figs.  86,  87  and  84)  step  by  step  over  the  contacts,  waiting 
a  few  seconds  on  each  contact  for  the  motor  speed  to  accelerate. 
If  this  process  is  performed  too  quickly  the  motor  may  be  in- 
jured by  excessive  current;  if  too  slowly,  the  rheostat  may  be 
injured.  If  the  motor  fails  to  start  on  the  first  step,  move 
promptly  to  the  second  step  and  if  necessary  to  the  third,  but 
no  farther.  If  no  start  is  made  when  the  third  step  is  reached, 


SEC.  3] 


DIRECT-CURRENT  MOTORS 


73 


open  the  line  switch  at  once,  allow  the  starter  handle  to  return 
to  the  off  position,  and  look  for  faulty  connections,  overload, 


AfctnpTfp 


Connection  Diagram. 


FIG.  85.  —  Direct-current  motor  starting  rheostat. 

etc.  The  time  of  starting  a  motor  with  full-load  torque  should 
not,  as  a  general  thing,  exceed  15  sec.  for  motors  of  5  h.p.  and 
lesser  output,  and  30  sec.  for  those  of  greater  output. 


FIG.  86. — Wiring  connections  for  a  shunt  motor  and  its  starting  rheostat. 

114.  In  Stopping  a  Direct-current  Motor,  open  the  line 
switch.     The  starting-rheostat  arm  will  return  automatically 


Compound'  Wound 
otor-.i 
~~~(~Shunt~Fieict      ' 


•minal    J^r Armature'-     ! 
Board-'       «• >-- J 


FIG.  87. — Wiring  connections  for  a  compound-wound  motor  and   its 

rheostat. 

to  the  off  position.     Never  force  the  operating  arm  of  any 
automatic-starting  rheostat  back  to  the  off  position. 


74 


ELECTRICAL  MACHINERY 


[ART.  115 


115.  Starting  Rheostats  for  Shunt-,  Compound-  and  Series- 
wound  Direct-current  Motors  vary  somewhat  in  detail,  design, 
and  method  of  connection  with  the  ideas  of  the  different 
manufacturers.  The  rheostat  shown  in  Fig.  85  is  fairly  typical 
of  those  for  starting  motors  of  outputs  up  to  120  h.p.  Enclosed 

starting  rheostats  (Fig.  88)  may 
frequently  be  employed  to  ad- 
vantage. 

116.  The  Low-voltage  Release 
Device    on  a   Starting  Rheostat 
consists  of  a  spring,  which  tends 
to  return  the  operating  arm  to  the 
off  position, and  an  electromagnet, 
which,  under  conditions  of  normal 
voltage,  holds  the  operating  arm 
in  the  running  position.     The  coil 
of  this  magnet  is   regularly  con- 
nected across  the  circuit  with  a 
protecting  resistance  in  series,  but 
can  be  connected  in  series  with 
the  shunt  field  of  the  motor  if 
specially  required.     If  the  voltage 
drops     below     a     predetermined 
value,  the  arm   is   released    and 
returned  by  the  spring  to  the  off 
position. 

117.  Arcing  Devices  on  Start- 
FIG.  88.-Connections    of    a  inS    Rheostats.— Arcing  tips  con- 
drum  controller  with  a  com-  sisting  of  pivoted  fingers  are  some- 
pound-wound  motor.                    timeg    mounted    near    the    point 

where  the  circuit  is  opened.  In  passing  to  the  off  position 
a  lug  on  the  end  of  the  arm  strikes  and  deflects  the  tip, 
which  is  in  electrical  connection  with  the  first  stationary  con- 
tact; the  current  is  diverted  to  the  tip,  which  snaps  back  when 
released  and  opens  the  circuit  very  quickly,  thus  rupturing 
the  arc.  Blow-out  coils  can,  where  necessary,  be  mounted  be- 
hind the  first  contact  and  will  disrupt  any  arc  formed  in  open- 
ing the  circuit. 


SEC.  3] 


DIRECT-CURRENT  MOTORS 


75 


118.  Overload  Release  Device  on  Starting  Rheostats. — This 
device,  which  is  not  illustrated,  includes  an  electromagnet, 
which,  in  case  of  over-  Gnuiti 


load,  attracts  its  arma- 
ture and  forces  an  in- 
sulating wedge  between 
two  contacts,  separat- 
ing them  and  thereby 
opening  the  circuit  of 
the  low- volt  age  release 
magnet.  The  operat- 
ing arm  returns  im- 
mediately to  the  off 
position.  With  some 
devices,  the  attraction 
of  the  armature  closes 


Line  Terminals^.  _ 


fiat 


FocRear  Connection. 


•Starter-.-  •••*. 


ForFront  Connection. 


FIG.  89. — Direct-current  motor  starting 
panels. 


t  Line-   ~ 


Fuse- 


Jtarfer- 


two  contacts  which  places  a  short-circuit  around  the  low- 
voltage  release  magnet,  thereby 
deenergizing  it  and  permitting  the 
operating  arm  to  return  to  the  off 
position.  It  should  be  noted  that 
the  National  Electrical  Code  rules 
require  the  use  of  fuses  or  circuit- 
breakers  (see  Art.  120  with  each 
rheostat  even  though  it  be  equipped 
with  an  overload  release  of  this 
nature. 

118a.  Starting  Panels  for  Direct- 
current  Motors  are  shown  in  Figs. 
89  and  90.  Panels,  of  which  the 
illustrations  are  typical,  are  very 
desirable  in  that  they  concentrate 
all  of  the  apparatus  for  the  motor's 
Series  ffe/d  control  at  one  point  and  greatly 

FIG.  90.— Wiring  diagram  of      .       vr      ,,          .  .  ,™  £ 

typical  starting  panel.         simplify  the  wiring.     Where  such 

a  panel  is  used,  it  is  merely  neces- 
sary to  run  the  two  line  wires  to  the  line  terminals  of  the 
panel,  run  the  three  leads  between  the  motor  and  the  panel 


unt 
Field 


76  ELECTRICAL  MACHINERY  [ART.  119 

and  the  installation  is  ready  for  operation.  The  designs  of 
different  manufacturers  vary  in  details.  The  panels  can  be 
obtained  for  either  front  or  rear  connection  and  with  circuit- 
breakers  or  fuses  for  over-load  protection.  Which  is  preferable 
is  determined  by  the  characteristics  of  the  installation  in 
question. 

119.  The  Advantages  and  Disadvantages  of  Fuses  vs.  Cir- 
cuit-breakers may  be  summed  thus:  (J.)  Fuses  have  a  time 
element  that  unmodified  circuit-breakers  do  not  have;  that  is, 
fuses  will  not  open  an  overloaded  circuit  as  quickly  as  will 
ordinary  circuit-breakers.    For  this  reason  fuses  may  be  pref- 
erable   for  motors  that  are  liable  to  very  brief  overloads, 
especially  where  expert  supervision  of  electrical  apparatus  is 
maintained,  as  in  large  mills  and  factories.     A  supply  of  extra 
fuses  must  be  kept  available.     Where  there  are  many  fuse 
replacements  the  cost  of  fuse  renewals  is  considerable.     (2) 
Circuit-breakers  can  be  reset  in  less  time  and  -with  less  trouble 
than  is  required  to  replace  blown  fuses,  and  no  extra  parts  are 
required.     Circuit-breakers  may  therefore  be  preferable  where 
time  saving  is  an  important  consideration.     The  first  cost  of 
the  circuit-breaker  equipment  is  more  than  that  for  fuses,  but 
for  severe  service  the  circuit-breakers  are  much  the  cheaper 
in  the  long  run. 

120.  The  National  Electrical  Code  Rules  Require  That  Each 
Motor  and  Its  Starter  be  Protected  by  fuses  or  a  circuit-breaker 
and  controlled  by  a  switch  which  must  plainly  indicate  whether 
on  or  off.     The  switch  and  cut-out  (fuses  or  circuit-breaker) 
are,  preferably,  located  near  the  motor  and  in  plain  sight  of 
it.     All  wiring  should  be  neat  and  workmanlike  and  the  wires 
should  be  run  in  conduit  wherever  possible.     For  further  in- 
formation relating  to  the  "Code"  requirements  (governing  the 
installation  of  motor-control  apparatus)  and  the  reasons  and 
explanations  therefor,  see  the  author's  WIRING  FOB  LIGHT  AND 
POWER. 

121.  In  a  Series-wound  Motor  the  Speed  Varies  Inversely 
as  the    Load    (Fig.   84)— the  lighter  the  load   the    higher 
the  speed.      See  also  Art.  59  under  "Direct-current  Motors 
and  Generators."     A  series-wound  motor  of  any  size,  when 


SEC.  3]  DIRECT-CURRENT  MOTORS  77 

supplied  with  full  voltage  under  no-load,  or  a  very  light 
load,  will  "run  away"  just  as  will  a  steam  engine  without 
a  governor  when  given  an  open  throttle.  For  a  given  load, 
a  series-wound  motor  with  its  rheostat  in  series  draws  the 
same  current  irrespective  of  the  speed  and  for  a  given  load 
the  speed  varies  directly  as  the  voltage.  The  speed  at  a 
given  load  may  be  varied  by  varying  the  resistance  in  the 
motor  circuit;  in  the  meantime  if  the  load  on  the  motor 
be  constant  the  current  drawn  from  the  line  will  be  constant 
regardless  of  the  speed. 

122.  Shunting  the  Field  of  a  Series  Motor.— The  above 
statements  relate  to  the  use  of  a  rheostat  in  series  with  a 
series-wound  motor.     If  a  resistance  or  rheostat  be  placed  in 
parallel  with  the  field  of  a  series-wound  motor  the  speed  will 
be  increased  instead  of  decreased  at  a  given  load.     This  is 
known  as    shunting  the  field    of   the    motor.     This    shunt 
would  never  be  applied  till  the  motor  has  been  brought  up 
to  normal  full  speed  by  cutting  out   the  starting  resistance. 
With  a  "shunted  field"  a  motor  drives  a  load  at  a  speed 
higher  than  normal  and  therefore  requires  a  correspondingly 
increased  current. 

123.  Shunted  Armature  Connection  of  a  Series  Motor.— If 
a  resistance  is  placed  in  parallel  with  the  armature  of  a  series 
motor,  the  motor  will  operate  at  less  than  normal  speed  when 
all  the  starting  resistance  has  been  cut  out.     This  connection 
is  known  as  a  "shunted  armature  connection"  and  is  useful 
where  a  low  speed  is  desired  at  light  loads  and  is  particularly 
useful  in  some  cases  where  the  load  becomes  a  negative  one, 
that  is,  where  the  load  tends  to  overhaul  the  motor,  as  in  low- 
ering a  heavy  weight. 

124.  Speed  Control  of  Shunt-wound  Motors. — A  shunt- 
wound  motor  (Fig.  86)  unlike  a  series  motor,  when  supplied 
with  full  voltage,  maintains  practically  a  constant  speed  re- 
gardless of  variations  in  load  within  the  limits  of  its  capacity. 
See    also  Art.    63   under    "Direct-current    Generators    and 
Motors."     It  automatically  acts  like  a  steam  engine  having 
a  very  efficient  governor.     The  speed  of  a  shunt-wound  motor 
may  be  decreased  below  normal  by  a  rheostatic  controller  in 


78 


ELECTRICAL  MACHINERY 


[ART.  125 


series  with  its  armature  and  may  be  increased  above  normal 
by  means  of  a  rheostat  in  series  with  its  field  winding.  The 
latter  rheostat  is  known  as  a  "  field  rheostat,"  and,  to  be  ef- 
fective, must  have  a  high  resistance  owing  to  the  small  cur- 
rent which  flows  through  the  shunt-field  winding. 

125.  Speed  Control  of  Compound-wound  Motors. — A  com- 
pound-wound motor  (Fig.  87)  is  a  hybrid  between  a  series  and 
shunt-wound  motor  and  its  characteristics  are  likewise  of  a 
hybrid  nature.  See  also  Art.  67  under  "  Direct-current  Gen- 
erators and  Motors."  A  compound-wound  motor  will  not 
"run  away"  under  no-load  as  will  a  series  motor,  but  its  speed 
decreases  as  the  load  increases,  though  not  so  rapidly  as  is 


•lock in 


Field 
Resistor 
Buttons 


Small  Capacity. 


Medium  Capacity. 


FIG.  91. — Machine-tool  controller. 

the  case  with  a  series-wound  motor.  The  characteristics  of 
the  compound-wound  motor  render  it  particularly  valuable  in 
cases  where  the  load  is  subject  to  wide  variation.  It  will  give 
a  strong  torque  in  starting  and  driving  heavy  loads  and  will 
not  race  dangerously  when  the  load  is  suddenly  relieved. 

126.  The  Speed  of  a  Compound-wound  Motor  may  be 
reduced  below  normal  by  means  of  a  rheostat  in  the  circuit 
of  its  armature.     The  speed  may  be  increased  above  -normal 
by  shunting  and  even  short-circuiting  the  series-field  winding, 
and  may  be  still  further  increased  by  means  of  a  field  rheostat 
in  series  with  the  shunt-field  winding. 

127.  Rotary,  Drum  or  Machine-tool  Type  Controllers  for 
Direct-current   Motors. — Although   controllers   of  this  type 
(Figs.  91.  92  and  93)  find  their  most  frequent  applications  on 


SEC.  3] 


DIRECT-CURRENT  MOTORS 


79 


'When  Used  wrth  Shunt  Motor 


FIG.  92. — Connection  diagram  for  machine-tool  controller  of  small 
capacity  using  revolving  arms  for  both  armature  and  field  control. 


Top  ^ 


Sfiantfld. 

B. 
When  Used  with  Irrterpote  Motor 


When  Used  with  Compound  Motor. 


Corrtroller  Connections. 

(Plan  View  of  face  Plate  as  Seen 

from  Handle  End.) 


FIG.  93. — Connection  diagram  for  machine-tool  controller  of  medium 
capacity  using  drum  for  armature  control  and  revolving  arm  for  field 
control. 


80  ELECTRICAL  MACHINERY  [ART.  128 

machine  tools,  they  are  very  desirable  for  any  service  where 
the  work  is  severe  and  where  the  expense  of  an  enclosed  con- 
troller is  justified.  Machine-tool  work  usually  requires  a  com- 
bination starting  and  speed-regulating  controller,  that  is,  one 
whereby  the  motor  is  started  by  cutting  out  armature  resist- 
ance. After  the  motor  is  started  its  speed  is  regulated  by 
varying  the  amount  of  resistance  in  series  with  the  shunt  fields 
These  controllers  can  be  purchased  for  this  service  and  for 
control  or  starting  service  of  practically  any  type.  The  methods 
of  construction  and  connection  are  so  numerous  that  only  one 
type  of  drum  controller,  one  which  is  used  for  machine-tool 
service,  will  be  described  here. 

128.  Advantages  of  Controllers  of  the  Drum  Type  are  that 
the  contacts  and  arm  are  entirely  enclosed  and  that  the  move- 
ment of  a  single  handle  in  one  direction  or  the  other  starts 
the  motor  in  a  corresponding  direction  and  brings  it  to  the 
running  speed  desired.     The  operating  arm  remains  securely 
locked  at  the  proper  notch  until  released  by  the  operator  by 
pressing  a  button  in  the  handle. 

EXAMPLE. — There  are  two  switching  devices  in  the  controller  shown  in 
Figs.  92  and  93.  One  connects  to  the  armature  or  starting  resistor  and 
the  other  connects  to  the  field  control  resistor.  Both  switching  devices 
are  operated  by  the  same  handle.  In  drum  controllers  of  small  capacity 
the  armature  switching  device  consists  of  an  arm  passing  over  contact 
buttons  and  all  of  the  resistors  are  mounted  within  the  drum ;  that  is, 
the  controller  is  self-contained.  In  controllers  of  large  capacity,  the 
armature  resistance  is  cut  in  and  out  by  a  rotating  drum  similar  to  that 
used  in  street-railway  service  and  all  of  the  resistors  are  mounted  ex- 
ternal to  the  controller.  The  field  resistance  is  cut  in  and  out  by  a 
rotating  arm  passing  over  contact  buttons  in  all  but  the  largest  con- 
trollers for  which  a  drum  is  used.  Arc  shields  between  drum  segments 
and  blowout  coils  are  provided  where  necessary.  The  controllers  can 
be  arranged  to  provide  dynamic  braking.  Speed  ranges  of  from  1  to  2 
to,  possibly,  1  to  6  are  usually  provided. 

129.  Operation  of  Drum-type  Controllers  (see  Figs.  92  and 
93). — Continuous  movement  of  the  operating  handle  in  either 
direction  first  starts  the  motor  in  the  corresponding  direction 
of  rotation,  then  cuts  out  the  starting  resistance,  and  finally 
cuts  in  the  field  resistance  until  the  desired  running  speed  is 


SEC.  3] 


DIRECT-CURRENT  MOTORS 


81 


reached.  The  handle  should  be  moved  over  the  starting 
notches  in  not  over  15  sec.  for  motors  of  possibly  10  h.p. 
capacity  and  in  not  over  30  sec.  for  larger  motors.  The  start- 
ing resistance  should  not  be  used  for  speed  control.  For  a 
quick  stop  when  operating  with  weakened  field,  move  the 
handle  quickly  to  the  first  running  notch,  hold  it  there  mo- 
mentarily and  then  move  it  to  the  off  position;  the  application 
of  full  field  strength  when  the  speed  is  high  causes  dynamic 
braking,  thus  checking  the  speed  quickly  and  without  shock. 
For  a  very  quick  emergency  stop,  the  handle  can  be  moved 
to  the  first  reversing  notch  after  checking  the  speed  by  dynamic 


Field 


Armature. 


Assembly 


Wiring    Diagram 


FIG.  94. — Non-automatic    starting    and    speed-adjusting    rheostat. 

braking,  but  this  operation  causes  severe  mechanical  and 
electrical  stresses;  and  should  never  be  carried  beyond  the  first 
notch.  When  the  motor  is  to  be  at  rest  for  any  considerable 
length  of  time,  open  the  line  switch/ 

130.  A  Non-automatic  Starting  and  Speed-adjusting  Rheo- 
stat for  Direct-current  Motors  is  shown  in  Fig.  94.  This 
device  has  no  low-voltage  or  overload  protection,  hence  is 
suitable  only  for  applications  where  skilled  attendance  is  avail- 
able. The  operating  arm  makes  contact  as  it  is  revolved 
between  the  circular  bars  and  the  resistance  contact  buttons. 
There  are  a  number  of  field-control  steps,  hence  close  speed 
adjustment  over  a  considerable  range  can  be  obtained.  The 
contact  buttons  of  the  inner  circular  segment  are  connected 
to  the  starting  resistor  and  the  contacts  of  the  outer  circle  are 
connected  with  the  running  resistor.  A  reading  of  the  follow- 


82 


ELECTRICAL  MACHINERY 


[ART.  131 


ing  paragraph  describing  the  operation   of  the   device  will 
render  clear  the  principles  involved. 

131.  Operation  of  a  Non-automatic  Starting  and  Speed- 
adjusting  Rheostat*  (Fig.  94). — To  start  the  motor,  close  the 
line  switch  or  circuit-breaker  and  move  the  operating  arm  of 
the  rheostat  over  the  starting  buttons  to  the  first  running 

position  (the  point  where  the  two 
bar  contacts  overlap).  A  motor 
starting  with  full-load  torque 
should  be  brought  to  this  point 
in  approximately  15  sec.  Further 
movement  of  the  operating  arm 
increases  the  motor  speed  by  field 
control.  The  motor  can  be  oper- 
ated continuously  with  the  arm 
on  any  field  contact  button,  but 
with  rheostats  of  this  design  must 
not  be  allowed  to  run  on  any 
starting  button.  To  stop  the 
motor,  open  the  line  switch  or 
circuit-breaker  and  move  the  rheo- 
stat arm  to  the  off  position.  The 
latter  movement  must  not  be  for- 
gotten, since  this  rheostat  has  no 
automatic  features.  To  protect 
the  motor  in  case  of  failure  of 
the  power  supply  and  its  subse- 
quent return  after  the  motor  has 
stopped,  a  low-voltage  release 
circuit-breaker  should  be  installed 
in  series  with  each  rheostat.  The  rheostat  handle  must  be  in 
the  off  position  before  the  circuit-breaker  is  closed. 

132.  A  Regulating  Controller  for  a  Direct-current  Series 
Motor  is  usually  connected  substantially  as  diagrammed  in  Fig. 
95.  A  non-reversing  controller,  C,  is  shown.  Hence,  when  it 
is  desired  to  reverse  the  direction  of  rotation  of  the  motor  the 
reversing  switch,  S,  must  be  thrown  to  the  reverse  position. 

*  Weatinghouse  Elec.  &  Manfg.  Co. 


FIG.  95. — Series  motor  con- 
trolled with  a  drum  controller. 


SEC.  3] 


DIRECT-CURRENT  MOTORS 


83 


The  circuit-breaker  or  buses  are  shown  at  B  and  the  main 
switch  at  M. 

133.  The  Connections  of  a  Regulating  Controller  for  a  Com- 
pound-wound Motor  are  shown  in  Fig.  88.     The  external  con- 
nections would  be  the  same  for  either  a  shunt-  or  compound- 
wound  motor  of  either  the  interpole  or  non-interpole  types. 
The  overload  protection  and  main  switch  are  located  as  in  the 
previous  example. 

134.  The  Principle  of  the  Automatic  Starter  is  illustrated 
diagrammatically  in  Fig.  96.     Starters  of  some  types  employ 
the  solenoid  arrangement  illustrated,  while  others  (Fig.  99) 
involve   a   series   of   contactors    or    automatically    operated 


FIG.  96. — The  elements  of  one  type  of  automatic  starter. 

switches  which  cut  out  the  starting  resistance  as  the  motor 
speed  increases.  Where  a  solenoid-operated  type  of  starter 
similar  to  that  shown  in  the  illustration  is  used,  a  master 
switch,  M,  or  push  button  is  utilized  to  close  the  control  cir- 
cuit through  an  auxiliary  switch.  When  the  auxiliary  switch, 
C,  is  thereby  closed,  this  completes  the  circuit  through  the 
solenoid,  S.  Thus  the  solenoid  is  energized  and  the  core  is 
pulled  up  into  it,  which  cuts  out  of  circuit  as  the  cere  rises  the 
resistor  sections  of  the  starting  rheostat.  A  dash  pot,  D,  is 
provided,  the  piston  in  which  is  attached  to  the  solenoid  core. 
Thereby  the  core  is  prevented  from  rising  abruptly  and  cutting 
out  the  resistor  sections  too  rapidly. 


84 


ELECTRICAL  MACHINERY 


(ART.  135 


135.  Multi-switch  Starters  for  Direct-current  Motors  are 

used  for  motors  of  large  output  and  for  motors  of  medium  out- 
put that  start  under  severe  overloads.  Starters  of  this  type 
are  built  of  capacities  of  about  50  h.p.  and  upward.  Fig.  97 
shows  a  typical  starter  of  this  type.  In  the  starter  shown, 
when  each  switch  is  closed  it  compresses  a  spring  which  insures 
firm  contact  between  the  copper  block  contacts.  The  first 
switch  of  single-pole  starters  of  the  design  illustrated  and  the 
first  two  switches  of  double-pole  starters,  close  and  open  the 
circuit.  These  switches  are  provided  with  arc-shields  and 
blow-out  coils.  A  mechanical  interlocking  device  makes  it 
impossible  to  close  the  switches  in  any  but  the  proper  order. 


pitches.   / 


Resistance..    Blow- 


Arm 


LugAtwchcs 
Con  fact  B 
when  Swilch 
is  Closed. 


Pendant 
Switth- 

Front  Elevation.  Wiring    Diagram. 

FIG.  97. — Multi-switch  starter  (single-pole  starter  is  shown). 

Each  starter  is  equipped  with  an  overload  release  and  a 
low-voltage  release,  which  throw  open  all  the  switches  in  event 
of  an  overload  or  a  failure  of  voltage.  Both  devices  are  effect- 
ive while  the  motor  is  being  started,  and  the  tripping  point 
of  each  is  adjustable  over  a  range.  The  overload  release  can 
be  tripped  by  hand.  In  order  to  insure  the  closing  of  all  the 
switches  a  pendant  switch  in  series  with  the  low-voltage  release 
coil  must  be  held  closed  until  the  last  switch  of  the  starter 
is  closed;  if  this  button  is  released  before  the  last  switch  is 
closed,  all  the  switches  promptly  open.  The  last  switch  auto- 
matically closes  the  release-coil  circuit. 

A  field  relay  switch  is  sometimes  provided  for  use  in  con- 
nection with  a  separate  speed-adjusting  field  rheostat.  This 
switch  short-circuits  the  field  rheostat  during  the  acceleration 


SEC.  3]  DIRECT-CURRENT  MOTORS  85 

of  the  motor,  so  that  the  motor  is  always  started  with  full  field 
strength  regardless  of  the  position  of  the  rheostat  arm.  If 
the  field  rheostat  arm  is  at  the  off  position,  the  short-circuit 
is  automatically  removed  when  the  motor  reaches  full  speed, 
but  if  there  is  more  resistance  in  series  with  the  motor  shunt 
field  than  would  be  safe  to  insert  in  one  step  the  field  rheostat 
arm  must  be  first  moved  to  the  off  position  before  the  rheostat 
is  available  for  speed  adjustment. 

136.  In   Starting    a  Motor  with   a   Multi-switch   Starter 
(Fig.  97)  close  the  switches,  one  at  a  time,  in  regular  con- 
secutive order.     With  the   single-pole  type  starter  the  first 
switch  closes  the  armature  circuit  with  all  the  resistance  in 
series  and  connects  the  shunt  field  of  shunt  and  compound- 
wound  motors  directly  across  the  line;  each  succeeding  switch 
short-circuits  a   section  of   resistance.     In    the    double-pole 
type  the  first  two  switches  must  be  closed  in  order  to  admit 
current  to  the  motor.     With  full-load  torque,   the    motor 
should  be  started  in  1  min.;  with  50  per  cent,  overload,  in 
30   sec.     The  motor  is  stopped  by  tripping   the    overload 
release. 

137.  Magnet-switch  Controllers  (Figs.  98  and  99)  consist 
essentially    of    a    group    of     electromagnetically    actuated 
switches.     Magnet  switches  can  be  arranged  into  an  almost 
innumerable  number  of  combinations  for  different  services. 

Only  one  arrangement  which  will  illustrate  the  principles 
involved  will  be  treated  here.  For  further  information  see 
manufacturers'  catalogues.  The  switches  are  operated  by 
shunt  magnet  coils.  Their  action  can  be  manually  or  auto- 
matically governed.  Fig.  98  gives  cross-sectional  views  of 
two  kinds  of  magnet  switches.  The  rate  of  acceleration  is 
controlled  by  a  series  relay  the  operation  of  which  will  be 
understood  by  reference  to  Fig.  99.  This  diagram  shows 
a  compound-wound  motor  connected  to  a  three-point  mag- 
net-switch controller  with  a  one-point  master  switch.  The 
magnet  switches  are  shown  at  MI,  Mz  and  M 3,  the  main 
contacts  at  I,  II,  and  ///,  and  the  interlocking  contacts  at 
a-dj  b-b,  '/-/,  etc.  Main  circuits  are  shown  in  heavy  lines 
and  interlocking  circuits  in  light  lines. 


86 


ELECTRICAL  MACHINERY 


[ART.  137 


The  series  relay,  h,  is  a  small  electromagnet,  the  plunger 
of  which  carries  a  contact  disc  which  normally  spans  two 
stationary  contacts  s-s,  in  series  with  the  magnet  coil  inter- 
locking circuit.  The  magnet  coil  is  connected  in  series  with 
the  main  circuit  and  the  amount  of  current  required  to  lift 
the  plunger  can  be  adjusted  by  weights  on  the  plunger. 
Note  that  the  line  switch  in  the  diagram  does  not  close 
either  the  motor  or  the  magnet  switch  circuits.  Operation 


,-  Arcing  Contact  Pin 


Panel 


.•Imutattnq  lube 
Interlock  Spring 
'•.Interlock  Duct 
Inwla 


ts 

Blow-out 
Coil 

sfeflV 

fastener 
Magnet-Cap 

•Magnet  Coif 
5tationary 

Plunger 


Tube  of       , 
"  •  -  Interlock     Frame  Work 

Pr{r"*     Interlock  I 

FinqerSupporf 
^.Interlock 
Disci 


FIG.  98. — Sectional    elevation    of   two    typical   magnet   switches. 


does  not  begin  until  the  master  switch  is  closed.  Closing 
the  master  switch  connects  coil  MI  across  the  circuit;  the 
plunger  rises  closing  contact  I  and  bridging  a-a  with  disc  1. 
The  motor  starts  and  the  high  starting  current  causes  the 
series  relay  to  open  gap  s-s,  in  the  control  circuit.  As  soon 
as  the  motor  current  falls  to  a  point  predetermined  by  the 
relay  adjustment,  gap  s-s  is  again  closed  and  coil  M2  is  con- 


SEC.  3] 


DIRECT-CURRENT  MOTORS 


87 


nected  across  the  line  through  s-s,   a-a  and  c-c.     Contacts 


Armature-. 


'Shunt  Field 
ies  Fielaf 


fwwwwywwwvl  D 

Resistance    TR2  T^i 

EF        CH 


JL 


Master- 
Switch 


•Manet' 


FIG.  99. — Magnet  switch  controller  arranged  for  the  automatic  accelera- 
tion control  of  a  compound-wound  motor. 

77  are  then  closed;  interlocking  contacts  b-b  and  d-d  are 
bridged  and  gap  c-c  is  opened. 
Contacts  77  short-circuit  resistance 
Rz-Rij  causing  an  increase  in  the 
motor  current,  so  that  the  series 
relay  again  opens  s-s.  But  the 
opening  of  c-c  and  the  closing  of 
b-b  has  meanwhile  connected  M% 
across  the  circuit  independent  of 
the  relay,  so  that  though  the  relay 
can  delay  the  closing  of  a  magnet  Senes 
switch,  it  has  no  control  over  one 
already  closed.  When  the  start- 
ing current  decreases,  gap  s-s  is 
again  closed;  coil  M3  is  energized, 
closing  contacts  777  and  connect- 
ing the  motor  directly  across  the 
line.  The  gap  /-/  is  bridged  with  FIG.  100.— Assembly  of  a 
contact  4,  so  that  the  coil  Mz  is  Magnet  switch  controller  for 
.  *  .  J ,  ,  elevator  service  showing  how 

removed  from  any  further  control    the  switches  may  be  mounted. 

by  the  relay. 

If  the  voltage  fails  the  magnet  switches  drop  open.     On 


Main 
Switch 


Fuse 


88 


ELECTRICAL  MACHINERY 


[ART.   138 


return  of  the  voltage  (the  master  switch  remaining  closed), 
they  close  automatically  in  the  correct'  sequence  to  start 
and  accelerate  the  motor.  For  some  kinds  of  service  an 
overload  relay  is  used.  This  relay  is  similar  to  the  series  ac- 
celeration relay  in  operation  but  it  is  so  connected  that  all 
but  the  first  magnet  switch  drops  open  when  an  overload 
occurs.  The  motor  now  operates  slowly  with  all  the  resist- 
ance in  series  with  the  arma- 
ture until  the  overload  is  re- 
moved, after  which  the  open 
switches  close  as  in  starting. 
The  point  at  which  the  relay 
will  operate  is  adjustable  by 
weights  on  the  plunger. 

Of  the  many  applications 
of  magnet  switches  possibly 
the  most  important  are  the 
control  of  direct-current  ma- 
chine-tool motors  and  eleva- 
tor motors.  Fig.  100  shows 
a  front  view  of  a  magnetic- 
switch  elevator  controller. 

138.  Field  Relay  Switches 
are  required  where  separate 
.rheostats  are  used  for  starting 
and  controlling  the  speeds  of 
motors.  This  is  required  by 
a  National  Electrical  Code  rule 
to  prevent  the  possibility  of 

starting  a  motor  with  weakened  field.  The  switch,  shown  in 
Fig.  101,  mounted  under  the  starter  handle  accomplishes  this 
function  by  short-circuiting  the  field  rheostat  during  accelera- 
tion so  that  the  motor  must  always  start  with  full  field  re- 
gardless of  the  position  of  the  field  rheostat  arm.  The  switch 
shown,  or  a  similar  one,  can  be  applied  to  ordinary  starting 
and  speed-regulating  rheostats  and  generally  should  be 
mounted  on  the  rheostat  at  the  factory  of  the  firm  that 
furnishes  it. 


Series  Field 
FIG.  101.— Field  rheostat  relay 
switch. 


SEC.  3]  DIRECT-CURRENT  MOTORS  89 

139.  The  Field  Relay  Switch  Shown  Consists  of  a  small 
electromagnet,    a    pivoted    switch    bar,    and    a   stationary 
contact.     The    switch    bar    is    normally    held    away   from 
the  contact  by  a  helical  spring.     The  magnet-coil,  switch  bar, 
and  contact  are  in  series  with  a  circuit  that  parallels  the 
field  rheostat.     When  the   operating   arm   of   the    starting 
rheostat   is   moved   to    the    first   step,   a    pin    on    its    hub 
presses  the  relay  switch  bar  against  its  stationary  contact, 
thus  short-circuiting   the    field    rheostat.     As    the    arm   is 
turned  the  pin  on  the  starter  hub  soon  releases  the  relay 
switch  bar;  but    the   relay   electromagnet,    energized  when 
the    contacts    close,    holds    this    bar  temporarily    in   place. 
The  winding  of  the  relay  electromagnet  is  so  proportioned 
that  if  there  is  little  or  no  resistance  in  series  with  the  motor 
shunt  field,  the  relay  magnet  will  release  the  switch  bar  before 
the  motor  is  brought  to  full  speed,  leaving  the  field  rheostat 
available  for  speed  adjustment.     But  if  the  field  rheostat 
arm  is  turned  so  that  there  is  more  resistance  in  series  with 
the  shunt  field  than  would  be  safe  to  insert   in   one  step, 
the  electromagnet  will  keep  the  relay    switch   closed  until 
the  arm  of  the  field  rheostat  is  brought  back  toward  the  off 
position. 

140.  Starting  and  Speed-adjusting   (Field-control)  Rheo- 
stats    for    Direct-current    Shunt-    and    Compound-wound 
Motors. — There  are  as  many  and  more  designs  as  there  are 
manufacturers,  but  the  equipment  shown  in   Figs.   102   and 
103  is  typical  and  can  be  used  for  starting  and  regulating 
speed  in  non-reversing  services  where  speed  adjustment  by 
field  control  is  desirable.     Fig.  104  shows  the  application  of 
one  of  these  rheostats  to  a  compound-wound   commutating- 
pole  motor.     The  apparatus  is  so  arranged  that  the  motor  is 
always  started  with  full  field  strength.     In  case  of  failure 
of  the  voltage,  the  field  control  resistance  is  automatically 
short-circuited  and  the  motor  is  disconnected  from  the  line. 

140a.  Construction  of  a  Starting  and  Speed-adjusting 
Rheostat  (Fig.  103). — The  rheostat  consists  of  a  face  plate 
carrying  the  contacts,  operating  arms,  and  safety  devices, 
mounted  in  connection  with  two  resistors.  One  resistor  is 


90 


ELECTRICAL  MACHINERY 


[ART.  140a 


for  starting  and  one  is  for  adjusting  the  field  strength. 
The  face  plate  carries  three  rows  of  stationary  contacts. 
The  upper  row  is  connected  with  the  field  adjusting  re- 
sistor, the  second  row  with  the  starting  resistor;  and  the 
lower  row  contains  a  long  curved  segment  for  short- 
circuiting  the  field  resistance  in  starting.  A  contact  for 
short-circuiting  the  armature  resistance  when  the  arm  is  in 
the  running  position  is  sometimes  provided.  The  face  plate 
supports  two  arms,  an  operating  arm  and  a  short-circuiting 
arm,  pivoted  to  the  same  hub  and  arranged  so  that  they 
cannot  pass  each  other.  The  operating  arm  carries  the 


FIG.  102. — Starting  and  speed-ad- 
justing rheostat. 


ShuntField 
FIG.  103. — Wiring   diagram  for 
starting  and  speed-adjusting  rheo- 
stat. 


handle  and  two  contact  fingers,  one  for  the  starting  con- 
tacts and  the  other  for  the  field  contacts.  The  short- 
circuiting  arm  has  a  contact  finger  which  slides  over  the  con- 
tact bar,  short-circuiting  the  field  resistance  in  starting, 
and  the  armature  resistance  while  running.  In  some  designs 
this  arm  also  carries  laminated  copper  brushes  which  short- 
circuit  the  starting  resistance  when  the  arm  reaches  the 
running  position.  A  spring  tends  to  return  the  short-cir- 
cuiting arm  to  the  off  position. 

Under  conditions  of  normal  operation  the  short-circuiting 
arm  is  held  in  the  running  position  against  the  force  of  the 
spring  by  an  electromagnet  connected  across  the  line  in  series 


SEC.  3] 


DIRECT-CURRENT  MOTORS 


91 


with  a  protecting  resistance.  If  the  voltage  falls  below  a 
predetermined  point,  the  arm  is  released  and  returns  to  the 
off  position,  carrying  the  operating  arm  with  it.  Rheostats 
for  this  service  are  frequently  arranged  so  that  the  circuit 
is  opened  between  a  lug  on  the  operating  arm  and  a  small 
pivoted  finger  with  a  centering  spring  mounted  near  the  first 
starting  contact  and  connected  to  it  electrically.  The 
current  is  always  broken  abruptly  no  matter  how  slowly  the 


Shunt 


CommutaHng  -  Pole 
Winding 


Starting  ana 

Speed-Adjusting 

Rheostat\ 

I  R 


FIG.  104. — Connections  of  a  starting  and  speed-adjusting  rheostat  for 
a  shunt-wound,  commutating-pole  motor. 

arm  may  be  moved.  Blow-out  coils  are  sometimes  mounted 
on  the  rear  of  the  face  plate  to  disrupt  any  arc  that  may  form. 
An  overload  release  device  can  be  mounted  on  all  but 
the  largest  rheostats  of  this  type.  It  consists  of  an 
electromagnet  which,  in  event  of  an  overload,  opens  the 
low-voltage  magnet  circuit,  thus  releasing  the  short-circuiting 
arm.  The  tripping  point  is  adjustable.  The  National 
Electrical  Code  rules  require  the  use  of  a  circuit-breaker 
or  fuses  with  a  rheostat  equipped  with  an  overload  release 
of  this  character.* 


Westinghouse  Elec.  &  Manfg.  Co. 


92 


ELECTRICAL  MACHINERY 


[ART.  140b 


140b.  Operation  of  a  Starting  and  Speed-adjusting  Rheo- 
stat (Figs.  102  and  103). — The  motor  is  started  by 
moving  the  operating  arm  to  the  running  position,  stopping 
a  few  seconds  on  each  starting  contact  to  permit  the 
speed  to  accelerate.  The  retaining  magnet  holds  the  short- 
circuiting  arm  in  the  running  position  where  it  short-circuits 
the  starting  resistor.  The  operating  arm  is  then  moved 
back  over  the  field-resistance  contacts  until  the  desired 
speed  is  reached.  For  motors  starting  with  full-load  torque, 
the  time  of  acceleration  should  be  from  15  sec.  to  30  sec.. 


FIG.  105. — Method  of  connecting  two  motors  so  that  they  both  may  be 
controlled  by  one  set  of  starting  and  speed-regulating  equipment. 

depending  upon  the  capacity  of  the  motor.  To  stop  the 
motor,  open  the  line  switch.  Both  arms  then  return  to  the 
off  position  automatically. 

141.  The  Arrangement  of  One  Starting  and  Speed-adjusting 
Rheostat  for  the  Control  of  Two  Motors,  A  and  Bt  is  illus- 
trated in  Fig.  105.  This  arrangement  was  used*  to  provide 
emergency  service  on  the  other  motor  if  one  motor  failed.  By 
throwing  the  three-pole  double-throw  switch,  S,  the  control 

*  R.  L.  Hervey  in  POWER,  Sept.  19,  1916. 


SEC.  3] 


DIRECT-CURRENT  MOTORS 


93 


equipment  is  connected  to  either  motor.  The  resistance  of 
the  field  regulator  of  C  not  being  sufficient  to  increase  the 
motor  speed  as  desired,  an  additional  rheostat,  R,  was  inserted 
in  the  field  circuit.  The  single-pole  switch,  Sz,  was  so  connected 
that  R  could  be  shunted  out  of  circuit  if  necessary. 

142.  Armature  Control  Speed  Regulators  (Fig.  106)  are 
used  for  speed  reduction  with  shunt,  compound  or  series  motors 
in  non-reversing  service  where  the  torque  required  decreases 
with  the  speed  but  remains  constant  at  any  given  speed  as 
with  fans,  blowers  and  centrifugal  pumps.  They  can  also  be 
used  for  applications  where  the  torque  is  independent  of  the 
speed,  as  with  job  printing  presses.  However,  this  method 

Field  Terminal  ,  Armature  Terminal 


Fuses-, 


'""•• •'"  -Line  Terminals  Line  Switch-'  Armature  - 

Front  View.  Wi'ring  Diagram. 

FIG.  106. — Armature-control  speed  regulator. 

of  speed  control  is  not  suitable  for  such  applications  where 
there  is  operation  for  long  periods  at  reduced  speed,  since  such 
operation  is  not  economical.  It  is  not  possible,  where  the 
torque  varies,  to  obtain  constant  speed  with  these  controllers. 
143.  Construction  of  an  Armature  Control  Regulator. — In 
the  regulator  shown  the  low-voltage  release  consists  of  an 
electromagnet  enclosed  in  an  iron  shell,  a  sector  on  the  pivot 
end  of  the  operating  arm,  and  a  strong  spring  which  tends  to 
return  the  arm  to  the  off  position.  The  magnet  is  mounted 
directly  below  the  pivot  of  the  arm  and  its  coil  is  connected 
in  shunt  across  the  line  in  series  with  a  protecting*  resistance. 
When  the  magnet  is  energized  its  plunger  rises  and  forces  a 


94  ELECTRICAL  MACHINERY  [ART.  144 

steel  ball  into  one  of  a  series  of  depressions  in  the  sector  on  the 
arm  with  sufficient  force  to  hold  the  arm  against  the  action  of 
the  spring;  each  depression  corresponds  to  a  contact.  The 
arm  can  be  easily  moved  by  the  operator,  however,  as  the  ball 
rolls  when  the  arm  is  turned.  When  the  voltage  fails,  the 
magnet  plunger  falls  and  the  spring  throws  the  operating  arm 
to  the  off  position.  An  overload  release,  similar  to  that  de- 
scribed in  another  paragraph,  which  operates  by  opening  the 
low- volt  age  coil  circuit,  is  sometimes  furnished  on  regulators 
of  this  type.  Standard  commercial  rheostats  of  this  type  are 
designed  to  give  about  50  per  cent,  speed  reduction  on  the 
first  notch.  See  the  following  paragraph  on  operation  for 
further  information. 

144.  Operation  of  Armature  Control  Speed  Regulators  (Fig. 
106). — Forward  motion  of  the  operating  arm  starts  the  motor 
and  brings  it  gradually  to  maximum  speed.     Moving  the  arm 
over  the  first  few  contact  buttons  increases  the  shunt  field 
strength  if  the  motor  is  shunt  or  compound.     The  movement 
over  the  succeeding  buttons  cuts  out  armature  resistance  and 
permits  the  motor  to  speed  up. 

145.  Objections  to  Armature  Control.  * — (a)  Bulk  of  Rheostat. 
— This  may  not  be  very  objectionable  if  only  a  few  motors 
are  so  controlled,  but  for  a  number  the  extra  space  becomes  a 
factor,  and  in  many  cases  it  is  difficult  to  find  sufficient  room 
near  the  motor. 

(6)  Inefficiency  of  the  System. — The  same  amount  of  power 
is  supplied  at  all  speeds  but  at  low  speeds  only  a  small  part  of  it 
is  converted  into  useful  work,  the  balance  being  wasted  in 
the  rheostat  as  heat. 

(c)  Poor  Speed  Regulation  with  Varying  Speeds. — Since  the 
impressed  voltage  at  the  armature  terminals  is  equal  to  the 
line  voltage  minus  the  resistance  drop  in  the  rheostat  any 
change  in  the  current  drawn  by  the  motor  produces  a  change 
in  the  terminal  voltage,  the  counter  e.m.f.,  and  therefore  the 
speed. 

This  condition  is  illustrated  by  the  graphs  of  Fig.  107, 
which  are  plotted  from  data  obtained  from  a  4-h.p.  motor. 

*  Crocker  and  Arendta,  ELECTRIC  MOTORS. 


SEC.  3] 


DIRECT-CURRENT  MOTORS 


95 


With  all  of  the  armature  resistance  in  circuit  the  speed  at  full- 
load — 4  h.p. — is  750  r.p.m.  whereas  at  no-load  the  speed  is 
1,220  r.p.m.  That  is,  there  is  a  drop  in  speed  of  470  r.p.m. 
between  no-load  and  full-load,  hence  the  speed  regulation  is 
470  -*-  1,220  =  38.5  per  cent. 

146.  Crane  Controllers  for  Direct-current,  Series  and  Com- 
pound-wound Motors  are  usually  arranged  somewhat  as  indi- 
cated in  Fig.  108.  The  switching  device  consists  of  a  disc  of 
soapstone  or  other  fireproof  insulating  material  carrying  station- 
ary contact  pieces  and  a  pivoted  switch  arm  carrying  four  con- 


FIG.  107. — Graphs   showing   speed -load   characteristics  with   different 
percentages  of  armature  resistance  in  circuit. 

tactors.  Blow-out  coils  are  usually  provided  to  effectively  rup- 
ture the  arcs  that  form  when  the  contactors  pass  from  one  con- 
tact piece  to  the  next.  The  resistors  may  be  contained  in  the 
controller  base,  as  in  small  controllers,  or  may  be  arranged  for 
separate  mounting  as  in  large  ones.  In  Fig.  108  the  fine  lines 
within  the  circle  are  shading  lines  which  merely  indicate  that 
the  circle  is  a  soapstone  disc.  Only  the  heavy  lines  within  the 
circle  represents  electrical  connections.  Fig.  109  shows  two 
typical  controllers. 

Movement  of  the  controller  handle  in  either  direction  past 


96 


ELECTRICAL  MACHINERY 


[ART.   146 


the  off  position  starts  the  motor  in  the  corresponding  direction 
of  rotation.     At  each  step  a  section  of  resistance  is  short-cir- 


Armature 

r-SWN&F* 1 

Series  Field 


FIG.   108.— Connections  of  a  16-point  crane  controller  connected  to  a 

series  motor. 

cuited.  At  the  full-speed  positions  all  the  resistance  is  short- 
circuited.  Stops  prevent  over-running  past  the  full-speed 
positions.  Direct-current  crane  controllers  increase  or  decrease 


""•——•  Switch  Arm  • ,' 


Movable 

!  Contactors 


Stationary 
Contacts'" 


O    C    •     O    9    O    • 


FIG.  109. — Crane  controllers. 


the  amount  of  resistance  in  series  with  the  motor  and  thereby 
control  its  speed. 


SEC.  3]  DIRECT-CURRENT  MOTORS  97 

147.  Dynamic  Braking  of  Direct-current  Motors  is  effected 
by  allowing  a  motor  to  be  temporarily  driven  as  a  generator 
by  its  load.     The  mechanical  energy  of  the  moving  machinery 
or  descending  load  is  thus  converted  into  electrical  energy  and 
then  into  heat  which  is  dissipated  in  resistance.     The  result 
is  that  the  speed  of  the  motor  is  promptly  retarded.     The 
amount  of  braking  action  can  be  adjusted  by  varying  the  cur- 
rent flowing  in  the  motor  armature.     A  load  exercising  an  active 
torque  on  the  motor  armature,  such  as  an  elevator  car,  can- 
not be  brought  to  a  full  stop  by  this  method,  since  with  the 
decreasing  armature  speed  the  braking  action  also  decreases. 
For  final  stopping,  some  form  of  mechanical  brake,  which  acts 
automatically,  is  therefore  necessary. 

148.  Dynamic  Braking  is  Used  in  connection  with  motors 
for  elevators,  hoists,  cranes,  coal  and  ore  handling  machinery, 
railway  cars,  etc.     It  is  employed  for  reducing  the  motor  speed 
just  before  a  stop,  as  in  elevator  service;  or  for  controlling 
the  speed  of  moving  objects,  as  in  lowering  crane  loads,  re- 
tarding the  speed  of  the  cars  descending  grades,  and  the  like. 

149.  The  Principal  Advantages  of  Dynamic  Braking  are  the 
practical  absence  of  all  wear  and  tear  on  the  apparatus,  con- 
venience of  application,  and  ease,  accuracy,  and  certainty  of 
control.     In  dynamic  braking  with  a  properly  selected  motor, 
active  deterioration  is  limited  to  the  controller  contacts,  which 
can  be  arranged  for  quick,  easy,  and  inexpensive  renewal.     No 
special  or  additional  apparatus  is  required  for  braking  except 
the  resistance  which  can  be  placed  wherever  convenient  within 
a  reasonable  distance  from  the  motor.     The  braking  effect  can 
be  adjusted  with  great  accuracy  over  a  wide  range  by  varying 
the  armature  current  or  the  field  strength  by  means  of  suitable 
resistance.     In  some  instances,  notably  with  railroads,  dynamic 
braking  actually  returns  energy  to  the  circuit;  but  in  industrial 
service  the  energy  generated  is  usually  dissipated  by  resistance. 
In  electric  cars,  during  the  winter  months,  this  dynamic  brak- 
ing current  is  in  many  cases  used  in  the  heaters  for  warming 
the  cars. 

150.  Heating  with  Dynamic  Braking. — The  most  important 
limitation  to  the  use  of  dynamic  braking  is  the  heating  of  the 


98 


ELECTRICAL  MACHINERY 


[ART.  151 


motor  by  the  generated  currents.  For  simple  stopping  duty 
this  action  is  insignificant,  as  it  lasts  only  a  few  seconds;  but 
with  speed  control  in  lowering  a  load  by  dynamic  braking,  the 
generated  current  may  flow  for  an  extended  length  of  time  and 
the  heating  may  be  considerable,  especially  as  it  is  added  to 
the  heating  of  the  machine  when  operated  as  a  motor.  This 
additional  heating  effect  due  to  the  braking  current  must  be 
considered  in  selecting  the  motor. 

151.  Dynamic-braking  Connections. — Fig.  110  shows  by 
simple  diagrams  some  of  the  possible  connections.  Diagram 
I  shows  the  armature  of  a  shunt  motor  short-circuited  through 


Lt'ne 


Line 


Shu nf  Field* 


Armature 


Line 


I     Series  Field  Resist. 
Series  Field 


F90Q0VV 

Hire 

Brake\ 
WWW 

Resist  ^— '          Resist. 

H.  IV. 

FIG.  110. — Dynamic  braking  connections. 

a  brake  resistance,  the  field  remaining  across  the  line.  Dia- 
gram II  shows  the  armature  of  a  compound  motor  short- 
circuited  through  the  series  field  and  a  brake  resistance,  the 
shunt  field  remaining  across  the  line.  Diagram  ///  shows  the 
armature  of  a  series  motor  short-circuited  through  the  series 
field,  a  protecting  resistance  for  the  field,  and  a  brake  resistance 
— the  field  and  its  resistance  being  in  series  across  the  line. 
Diagram  IV  shows  the  armature  and  series  field  of  a  series 
motor  short-circuited  through  a  brake  resistance,  all  of  which 
are  entirely  disconnected  from  the  line. 

By  cutting  out  the  series  field  in  diagram  II  the  braking 


SEC.  3]  DIRECT-CURRENT  MOTORS  99 

effect  can  be  diminished,  the  connections  then  being  as  in  /. 
The  connections  shown  in  diagram  ///  are  generally  preferable 
for  series  motors  during  the  first  part  of  the  braking  operation, 
in  order  to  insure  building  up  as  a  generator.  As  soon  as  the 
generator  action  has  begun,  the  connections  can  be  changed 
to  those  shown  in  diagram  IV.  In  each  of  the  cases  shown 
by  the  four  diagrams  the  braking  effect  can  be  increased  by 
short-circuiting  sections  of  the  brake  resistance  and  thus  in- 
creasing the  armature  current. 

152.  The  Control  of  Large  Direct-current  Motors  with  an 
Equalizer  Flywheel  Motor-generator  Set  (Figs.  Ill  and 
112)  is  illustrated  diagramatically  in  Fig.  113.  Methods  of 


FIG.  111. — A  flywheel,  motor-generator  hoisting  set  installed  for  the 
North  Butte  Mining  Company.  The  motors  are  the  largest  in  the  world 
installed  in  mining  service  for  hoisting.  The  direct-current  motor  is 
an  1850  h.p.,  71  r.p.m.  machine.  The  alternating  current  motor  has 
a  capacity  of  1400  h.p.  The  direct-current  generator  has  an  output  of 
1500  kw.  The  outfit  hoists  as  much  as  10  tons  of  ore  per  trip.  See 
Fig.  112  for  motcr  and  hoists.  (Westinghouse  Electric  Co.) 

control  similar  to  that  suggested  have  been  employed  for 
mine-hoist  motors  and  for  rolling-mill  motors  having  capaci- 
ties as  great  as  15,000  h.p.  Energy  for  driving  the  roll  or 
hoist  motor  is  supplied  from  a  three-phase,  alternating- 
current  line,  but  this  alternating  current  does  not  directly 
drive  the  roll  motor,  R,  but  instead  merely  the  motor, 
M  (Fig.  113),  which  drives  the  flywheel  motor-generator  set. 
The  energy  to  the  roll  motor  is  supplied  directly  by  the 
direct-current  generator,  G,  of  this  set.  The  roll  motor,  R, 
is  controlled  by  varying  the  strength  and  direction  of  the 
field  current  of  the  direct-current  generator,  G,  which  field 


100 


ELECTRICAL  MACHINERY 


[ART.  152 


FIG.  112. — Variable  speed,  direct-current  motor  and  hoist  of  the  North 
Butte  equipment.  See  Fig.  Ill  for  the  flywheel,  motor-generator 
set. 


•AC.  Motor 


Reversing  f/'eM  Controller-  - 


Rheostat— "> 


FIG.  113. — Diagram   of   flywheel-motor-generator   control   for  a  large 
motor  operating  under  service  conditions, 


SEC.  3]  DIRECT-CURRENT  MOTORS  101 

current  is  supplied  by  exciter  E.  With  this  method,  to 
control  the  speed  of  and  reverse  the  roll  motor,  it  is  merely 
necessary  to  vary  or  reverse  the  field  current  of  the 
generator  by  manipulating  controller  C.  Dynamic  braking 
is  used  to  stop  the  roll  motor. 

153.  The  Function  of  the  Flywheel  of  the  Motor-generator 
Set  is  to  equalize  the  draft  of  energy  from  the  alternating- 
current  line,  L.     If  the  mill  motor   tends  to  draw  a  large 
current  from  the  line,  the  motor-generator  set  would  tend  to 
slow  down.     Then  the  flywheel  will  impart  its  stored  energy 
to  the  system  during  these  moments  of  heavy  load  on  the  roll 
motor.      When  the  peak  load  is  over,  the  alternating-current 
motor  will  again  attain  normal  speed  and  store  energy  in  the 
flywheel.      The    load    on    the    alternating-current    motor  is 
greater  than  that  on  the  roll  motor  when  the  roll  motor  is 
lightly   loaded.     But,   when   the    roll  motor,    R,  is  pulling 
its  peak  loads,  the    alternating-current   motor,  M,    carries 
the  smaller  load. 

154.  The   Function   of  the   Torque  Motor,  T  (Fig.  113), 
is  to  throw  the  peak  load  on  the  flywheel.      It  does  this  by 
introducing    resistance  into  the  rotor  circuit   of   the   large 
alternating-current  motor,   M,   thereby  reducing  its  speed. 
The  slip  regulator  is  merely  a  liquid  rheostat  having  three 
pairs  of  electrodes,  one    pair  connected  in  series  with  each 
phase  of  the  rotor  circuit  of  M .     The  movable  electrode  of 
each  pair  is  connected  to  an  arm  arranged  on  the  shaft  of 
the  small  torque  motor.      When  the  current  in  the  line,  L, 
tends  to  decrease  due  to  a  heavy  draft  of  energy  by  R,  the 
torque  motor  decreases  the  distance  between  the  electrodes 
and  M  increases  its  speed  accordingly. 

155.  A  Float  or  Tank    Switch  is  illustrated   in  Fig.  114. 
Devices  of  this  character  are  used  to  stop  and  start  pump 
motors,  such,  for   example,    as  those  used  for  draining  the 
sumps  in  basements  of  buildings.     As  the  water  in  the  pit 
rises,  the  rod,  R,  is  forced  upward  by  the  action  of  the  float, 
F,  on  the  surface  of  the  liquid  in  the  pit.     On  the  rod  are 
two  stops,  S  and  S'.     When  the  pit  is  almost  full,  the  stop, 
S',  will  push  against   the  switch  arm,   A,   and   when  this 


102. 


ELECTRICAL  MACHINERY 


[ART.  156 


arm  has  been  forced  to  move  a  certain  distance,  the  weighted 
tumbler,  TF,  flops  over  and  causes  the  switch  to  close.  This 
energizes  the  solenoid  of  the  automatic  starter  ordinarily 
used  with  such  motors  (Fig.  265)  and  the  motor  commences 
to  drive  its  pump,  which  raises  the  water  out  of  the  pit. 
As'  the  water  in  the  pit  is  lowered,  the  float  falls  and  finally 
stop  S  strikes  the  switch  arm,  the  control  circuit  is  opened 
and  the  motor  is  stopped. 


w 


-Rod 


Pressure 


FIG.  114. — Illustrating  the  mech- 
anism of  a  float  switch. 


FIG.  115. — An  automatic  starter 
arranged  in  combination  with  a 
pressure  regulator  for  controlling  a 
direct-current  motor. 


156.  The    Method   of    Connecting   an  Automatic   Starter 
for  a  Pressure-regulator-control,    Direct-current    Motor   is 
illustrated  in  Fig.  115.     Where  a  motor  is  to  be  controlled 
by  a  float  switch  the  general  scheme  of  connections  is  similar 
to  those  illustrated  in  Fig.  265,  which  shows  the  connections 
for  alternating-current  motors. 

157.  Field-discharge  Switches  and  Resistors  should  be  pro- 
vided in  connection  with  all  electrical  machines  having  field 
circuits,  of  considerable  inductance,  which  are  energized  by 
direct  current.     Unless  some  sort  of  resistor  path  is  provided 
for  the  dissipation  of  the  electro-magnetic  energy  stored  in  the 


SEC.  3]  DIRECT-CURRENT  MOTORS  103 

magnetic  field,  it  will  cause   destructive  arcing  at  the  field 

a  "• 


-Shunt  Field 
I-No  Discharge  Resistor 


— AAAAAAA — ' 

R,         v -Discharge  Resistor 
I-  Discharge  Resistor  Around 
Shunt  Fieli 


•ield  1-Field  Discharge  Switch 

FIG.  116. — Arrangement  of  discharge  resistors. 

switch  whenever  this  switch  is  opened.  The  voltage  to  which 
this  arcing  is  due  is  an  e.m.f. 
of  self-induction,  and  it  may  be 
(unless  a  dissipative  resistor 
is  provided)  great  enough  to 
puncture  the  insulation  on  the 
field  windings.  Fig.  116  illus- 
trates some  of  the  possible  con- 
nections as  applied  to  direct- 
current  motors,  but  the  general 
principle's  there  indicated  can, 
with  obvious  modifications,  be 
applied  to  the  shunt-field  cir- 
cuits of  all  direct-current  ma- 
chines and  to  the  field  circuits 
of  alternating-current  ma- 
chines. In  Fig.  116,  /,  there 
is  no  field  discharge  resistor  or 
switch,  hence  when  the  main 
switch,  Si,  is  opened,  the  field 
can  discharge  directly  through  swj^J 
the  armature.  Where  auto-  winding  terminals  and  B  connects 
matic-control  equipment  isto  -one  en<^  of  the  field  discharge 
used  with  motors  a  switch,  T, 


104  ELECTRICAL  MACHINERY  [ART.  157 

is  often  inserted  between  the  shunt  field  and  the  arma- 
ture and  a  field  discharge  resistor,  R,  is  then  arranged  across 
the  shunt  field  as  suggested  in  the  diagram.  Where  this  de- 
vice is  utilized,  there  is  a  constant  PR  loss  in  the  discharge 
resistor,  Ri,  so  long  as  the  voltage  is  impressed  across  the 
shunt  field.  A  shunt-field  resistor  may  have  a  relatively  high 
resistance,  hence  this  PR  loss  is  often  negligibly  small  and 
the  simplicity  of  the  scheme  of  //  may  justify  its  employment. 
At  ///,  a  field  discharge  switch,  £3  (Fig.  117),  is  employed 
which  cuts  the  discharge  resistor  in  circuit  with  the  shunt  field 
whenever  the  main  switch  is  opened. 


SECTION  4 

TROUBLES  OF  DIRECT -CURRENT  GENERATORS  AND 

MOTORS 

158.  Troubles  of  Direct-current  Generators  and  Motors.* — 

Since,  as  explained  in  Art.  52,  the  construction  of  direct-cur- 
rent generators  is  essentially  the  same  as  that  of  direct-current 
motors,  most  of  the  troubles  inherent  to  one  are  also  en- 
countered with  the  other — the  remedies  for  generator  troubles 
are  about  the  same  as  those  for  motor  troubles.  Therefore, 
while  some  of  the  matter  in  this  section  applies  specifically 
to  direct-current  motors  it  may  under  certain  conditions  also, 
with  equal  force,  relate  to  direct-current  generators. 

*  Considerable  of  the  material  under  this  heading  is  based  on  articles  which  have 
appeared  in  the  magazine  POWER  (McGraw-Hill  Publishing  Company,  New  York)  and 
on  that  in  the  book,  MOTOR  TROUBLES,  by  E.  B.  Raymond. 


105 


106 


ELECTRICAL  MACHINERY 


[ART.  159 


159.  Direct-current  Generator  and  Motor  Troubles. 

by  special  permission) 


(From  MACHINERY, 


Sparking  at  the  brushes 

Commutator  I  Brushes 

Not  set  diametrically 
opposite. 

A.   Should  have  been  set  properly  at  first,  by  counting 
bars,  or  by  measurement  on  the  commutator. 
B.    Can  be  done  if  necessary  while  running;  move  rocker 
until  brush  on  one  side  sparks  least,  then  adjust  other 
brushes  so  they  do  not  spark. 

1 

Not    set    at    neutral 
points. 

Move  rocker  back  and  forth  slowly  until  sparking 
stops. 

2 

Not  properly  trimmed. 

A.    Brushes  should  be  properly  trimmed  before  starting. 
If  there  are  two  or  more  brushes  one  may  be  re- 
moved and  retrimmed. 
B.    Clean  with  alcohol  or  ether,  then  grind  and  reset 
carefully.     See  lines  1,  4,  38. 

3 

Not  in  line. 

Adjust  each  brush  until  bearing  is  on  line  and  square 
on  commutator  bar,  bearing  evenly  the  whole  width. 
See  line  13  A. 

4 

Not  in  good  contact. 

A.   Clean  commutator  of  oil  and  grit.     See  that  brushes 
touch. 
B.    Adjust  tension  screws  and  springs  to  secure  light, 
firm  and  even  contact.     See  line  38  B. 

5 

Rough  ;       worn       in 
grooves    or    ridges  ; 
out  of  round. 

A.   Grind  with  fine  sandpaper  on  curved  block,   and 
polish  with  crocus  cloth.     Never  use  emery  in  any 
form. 
B.    If  too  bad  to  grind  down  turn  off  true  in  a  laths  or 
preferably  in  its  own  bearings,  with  a  light  tool  and 
rest,  a  light  cut;  running  slowly.     Note.  —  Armature 
should  have  Me  to  H-in.  end  motion  when  running, 
to   wear   commutator   evenly   and   smoothly.     See 
line  31. 

6 

High  bars. 

Set  "high  bar"  down  carefully  with  mallet  or  block 
of  wood,  then  clamp  tightly  end  nuts,  or  file,  grind 
or  turn  true.     A  high  bar  may  cause  singing.     See 
line  38. 

8 

Low  bars. 

Grind  or  turn  commutator  true  to  the  surface  of  the 
low  bars. 

9 

Weak  magnetic  field. 

A.    Broken  circuit  \  .     _  ,  ,      .,    /  Repair  if  external. 
B.    Short-circuit      /  1D                   J  I  Rewind  if  internal. 
C.    Machine  not  properly  wound,   or   without   proper 
amount  of  iron  —  no  remedy  but  to  rebuild  it. 

10 

SEC.  4]   DIRECT-CURRENT  GENERATORS  AND  MOTORS     107 


160.  Direct-current  Generator  and  Motor  Troubles.    (From  MACHINERY, 
by  special  permission) — Continued 


Excessive  load. 

A.    Reduce  number  of  lamps  and  load. 

11 

Ground  and  leak 

B.    Test  out,  locate,  and  repair. 

from    short-cir- 

0} 

| 

cuit  on  line. 

S 

3 

0> 

Dead     short-cir- 

C.   Note.  —  Dead  short-circuit  will  or  should  blow  safety 

o] 
S 

0 

cuit  on  line. 

fuse.     Shut  down,   locate   fault   and   repair   before 

el 

starting  again,  and  put  in  a  new  fuse. 

.S 

Excessive  voltage. 

D.    Use  proper  current  only,  and  with  proper  rheostat 

1 

and  controller  and  switch. 

s 

Excessive       am- 

E.   See  that  controller,   etc.,   are  suitable  with  ample 

> 

peres    on    con- 

resistance. 

'i 

c 

stant-current 

i 

g 

circuit. 

"8 

Friction. 

F.    Reduce  load  on  motor  to  its  rated  capacity  or  less. 

See  3  B  and  35,  36. 

Too    great    load 

G.    See  that  there  is  no  undue  friction  or  mechanical  re- 

1 

on  pulley. 

sistance  anywhere. 

1 

Short-circuited  coils. 

A.    Remove  copper  dust,  solder  or  other  metallic  contact 

12 

•a 

between  commutator  bars. 

B.    See  that  clamping  rings  are  perfectly  free,  and  in- 

sulated from  commutator  bars;  no  copper  dust,  car- 

bonized oil,  etc.,  to  cause  an  electrical  leak. 

w 

C.    Test  for  cross  connection  or  short-circuit,  and  if  such 

is  found  rewind  armature  to  correct. 

D.   See  that  brush  holders  are  perfectly  insulated.     No 

copper  dust,  carbon  dust,  oil  or  dust,  to  cause  an 

electrical  leak.     See  1,  2,  60. 

3 

H 

Broken  coils. 

A.    Bridge    the   break   temporarily   by   staggering   the 

13 

brushes,  until  machine  can  be  shut  down  (to  save 

£ 

bad  sparking)  and  then  repair. 

1 

B.    Shut  down  machine  if  possible,  and  repair  loose  or 

S 

broken  connection  to  commutator  bar. 

45 

C.    If  coil  is  broken  inside,  rewinding  is  the  only  sure 

remedy.     May  be  temporarily  repaired  by  connect- 

ing to  next  coil,  across  mica. 

D.   Solder    commutator    lugs    together,    or    put    in    a 

''jumper,"  and  cut  out,  and  leave  open  the  broken 

coil.     Be  careful  not  to  short-circuit  a  good  coil  in 

doing  this.     See  12. 

Cross  connections. 

Cross  connections  may  have  same  effect  as  short- 

14 

circuit,  treat  as  such,  see  12.     Each  coil  should  test 

complete  without  cross  and  no  ground. 

108 


ELECTRICAL  MACHINERY 


[ART.  161 


161.  Direct-current  Generator  and  Motor  Troubles.     (From  MACHINERY, 
by  special  permission) — Continued 


Heating  of  parts 

Armature 

Overloaded. 

Overload.  Too  many  amperes,  lights,  or  too  much 
power  being  taken  from  machine.  See  11,  12,  13,  14. 

Short-circuit. 

Short-circuited.  Generally  dirt,  etc.,  at  commu- 
tator bars.  See  11,  12,  13,  14. 

Broken  circuit. 

Broken  circuit.  Often  caused  by  a  loose  or  broken 
band.  See  11,  12,  13,  14. 

Cross  connection. 

Cross  connection.  Often  caused  by  a  loose  coil 
abrading  on  another  coil'  or  core.  See  11,  12,  13,  14. 

Moisture  in  coils. 

Dry  out  by  gentle  heat.  May  be  done  by  sending  a 
small  current  through,  or  causing  machine  to  gener- 
ate a  small  current  itself,  by  running  slowly. 

Eddy  currents  in  core. 

Iron  of  armature  hotter  than  coils  after  a  run. 
Faulty  construction.  Core  should  be  made  of  finely 
laminated  insulated  sheets.  No  remedy  but  to 
rebuild. 

Friction. 

Hot  boxes  or  journals  may  affect  armature.  See  23, 
33  below. 

£ 
1 

1 

fa 

Excessive          Shunt, 
current. 

A.  Decrease  voltage  at  terminals  by  reducing  speed. 
Increase  field  resistance  by  winding  on  more  wire, 
finer  wire,  or  putting  resistance  in  series  with  fields. 

Series. 

B.  Deprease  current  through  fields  by  shunt,  removing 
some  of  field  winding  or  rewind  with  coarser  wire. 

Note.  —  Excessive  current  may  be  from  a  short-circuit 
or  from  moisture  in  coils,  causing  a  leakage.  See 
10,  24. 

Eddy  currents. 

Pole  pieces  hotter  than  coils  after  short  run,  due  to 
faulty  construction,  or  fluctuating  current;  if  latter, 
regulate,  and  steady  current. 

Moisture  in  coils. 

Coils  show  less  than  normal  resistance,  may  cause 
short-circuit  or  body  contact  to  iron  of  dynamo. 
Dry  out  as  in  19.  See  also  22  note. 

I 

i 

« 

Not  sufficient  or  poor 
oil. 

A.   See  that  plenty  of  good  mineral  oil,  filtered  clean,  and 
free  from  grit,  feeds;  but  be  careful  that  it  does  not 
get  on  commutator  or  brush  holder.     See  12. 
B.    Cylinder  oil  or  vaseline  may  be  used  if  necessary  to 
complete  run,  mixed  with  sulphur  or  white  lead,  or 
hydrate  of  potash.     Then  clean  up  and  put  in  good 
order. 

SBC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     109 


162.  Direct-current  Generator  and  Motor  Troubles — Continued 


Heating  of  parts 

1  Bearings  | 

Dirt  or  grit  in  bear- 
ings. 

A.   Wash  out  grit  with  oil  while  running,  then  clean  up 
and  put  in  order.     Be  careful  about  flooding  commu- 
tator and  brush  holder. 
B.    Remove  caps  and  clean  and  polish  journals  and 
bearings  perfectly,  then  replace.     See  that  all  parts 
are  free  and  lubricate  well. 
C.    When  shut  down,  if  hot,  then  remove  bearings  and 
let  them  cool  naturally,  then  clean,  scrape  and  polish, 
assemble;  see  that  all  parts  are  free  and  lubricate  well. 

26 
27 

Rough  journals  or 
bearings. 

Smooth  and  polish  in  a  lathe,  removing  all  burrs, 
scratches,  tool  marks,  etc.,  and  rebabbitt  old  boxes 
and  fit  new  ones. 

Journals  too  tight  in 
bearings;  bent  shaft. 

Slacken  cap  bolts,  put  in  liners  and  retighten  till 
run  is  over,  then  scrape,  ream,  etc.,  as  may  be  needed, 
bend  or  turn  true  in  lathe  or  grinder. 
Possibly  a  new  box  or  shaft  will  be  needed. 

28 
29 

Bearings  out  of  line. 

Loosen  bearing  bolts,  line  up  and  block,  until  arma- 
ture is  in  center  of  pole  pieces,  ream  out  dowel  and 
bolt  holes  and  secure  in  new  position. 

30 

End  pressure  of  pul- 
ley hub  or  shaft 
collars. 

A.   See  that  foundation  is  level  and  armature  has  free 
end  motion. 
B.    If  there  is  no  end  motion,  file  or  turn  ends  of  boxes 
or  shoulders  on  shaft  to  provide  end  motion. 
C.    Then  line  up  shaft  and  belt,  so  that  there  is  no 
end  thrust  on  shaft,  but  that  the  armature  plays 
freely  endways  when  running. 

31 

Belt  too  tight. 

A.   Reduce  load  so  that  belt  may  be  loosened  and  yet 
not  slip.     Avoid  vertical  belts  if  possible. 
B.    Choose  larger  pulleys,  wider  and  longer  belts  with 
slack  side  on  top.     Vibrating  and  flapping  belts  cause 
winking  lamps. 

32 
33 

Armature  out  of 
center  of  pole  pieces. 

A.   Bearings  may  be  worn  out  and  need  replacing,  throw- 
ing armature  out  of  center.     See  36. 
B.    Center  armature  in  polar  space,  and  adjust  bearings 
to  suit.     See  30. 
C.    File  put  polar  space  to  give  equal  space  all  round. 
D.   Spring  pole  away  from  armature;  this  may  be  diffi- 
cult or  impossible  in  large  machines. 

110 


ELECTRICAL  MACHINERY 


[ART.   163 


163.  Direct-current  Generator  and  Motor  Troubles — Continued 


Noises 

Armature  or  pulley 
out  of  balance. 

Faulty  construction,  armature  and  pulley  should 
have  been  balanced  when  made.  May  be  helped  by 
balancing  on  knife  edges  now. 

34 

Armature  strikes  or 
rubs  pole  pieces. 

A.    Bend  or  press  down  any  projecting  wires,  and  secure 
with  tie  bands. 
B.    File  out  pole  pieces   where  armature    strikes.      See 
30,  33. 

35 

Collars  or  shoulders 
on  shaft  strike  or 
rub  box. 

Bearings  may  be  loose  or  worn  out.  Perhaps  new 
bearings  are  needed.  See  30,  31. 

36 

Loose  bolt  connection 
or  screws. 

See  that  all  bolts  and  screws  are  tight,  and  examine 
daily  to  keep  them  so. 

37 

Brushes  sing  or  hiss. 

A.   Apply  stearic  acid  (adamantine)  candle,  vaseline,  or 
cylinder  oil  to  commutator  and  wipe  off;  only  a 
trace  should  be  applied. 
B.    Move  brushes  in  and  out  of  holder  to  get  a  firm, 
smooth,   gentle  pressure,   free  from  hum   or  buzz. 
See  3,  6,  7,  8,  9,  31. 

38 

Flapping  of  belt. 

Use  an  endless  belt  if  possible,  if  a  laced  belt  must  be 
used,  have  square  ends  neatly  laced. 

39 

Slipping  of  belt  from 
overload. 

Tighten  belt  or  reduce  load.  See  32. 

40 
41 

Humming  of  arma- 
ture lugs  or  teeth. 

A.   Slope  end  of  pole  piece  so  that  armature  does  not 
pass  edges  all  at  once. 
B.    Decrease  magnetism  of  field,  or  increase  magnetic 
capacity  of  tooth. 

•2 
1 

Runs  too  fast 

Engine  fails  to  regu- 
late with  varying 
load. 

Adjust  governor  of  engine  to  regulate  properly,  from 
no-load  to  full-load,  or  get  a  better  engine. 

42 
43 

Series  motor,  too 
much  current,  and 
runs  away. 

A.    Series  motor  on  constant  current  —  (1)  Put  in  a  shunt 
and  regulate  to  proper  current;    (2)  use  regulator  or 
governor  to  control  magnetism  of  field  for  varying 
load. 
B.    Series  motor  on.  constant  potential  —  (1)  Insert  resist- 
ance and  reduce  current;  (2)  use  a  proper  regulator  or 
controlling  switch;  (3)  change  to  automatic  speed- 
regulating  motor. 

SEC.  4]   DIRECT-CURRENT  GENERATORS  AND  MOTORS     111 


164.  Direct-current  Generator  and  Motor  Troubles — Continued 


Field    rheo- 

A.   Adjust  field  rheostat  to  control  motor. 

1 

g 

stat        not 

-2 

properly  set. 

I 

£ 

Not    proper 

B.    Use  current  of  proper  voltage  and  no  other,  with  a 

g 

G 

current. 

proper  rheostat. 

3 

0 

Motor      not 

C.    Get  a  better  motor,  one  properly  designed  for  tn« 

PH 

3 

properly 

work. 

proportioned. 

Runs 

See     note      below 

45,  same  as  42;  46,  see  11  A;  47,  short-circuit  in  arma- 

too 

table. 

ture,  see  12;  48,  rubbing  armature,  see  35;  49,  fric- 

slow 

tion,  see  3  B;  50,  weak  magnetic  field,  see  10. 

' 

Great  overload. 

Open  switch,  find  and  repair  trouble.     Keep  switch 

See  11  F  and 

open  and  rheostat  "off"  to  see  if  everything  is  right. 

G. 

Excessive  fric- 

Shunt motor  on  constant  potential  circuit,  fuse  may 

tion.    See  25, 

blow  or  armature  burn  out. 

33,  35. 

Fuse  melted 

A.   Find  and  repair  trouble  after  opening  switch,  then 

or     switch 

put  in  fuse.     See  11  C. 

tS 

open. 

cS 
• 

e 

Broken  wire 

B.    Open  switch,  find  and  repair  trouble.      See  13. 

3 

a 

or    connec- 

° 

tion. 

2 

B 

Brushes  not 

C.    Open  switch  and  adjust.     See  5. 

0 

in  contact. 

a 

0 

'C 

Current  fails 

D.   Open  switch  and  return  starting  box  lever  to  off  posi- 

GO 

or   is   shut 

tion,  wait  for  current. 

off  at  sta- 

tion. 

Short-circuit  of 

Test  for  and  repair  if  possible.     Examine  insulation 

field. 

of  binding  posts  and  brush  holders. 

Short-circuit  of 

Poor  insulation,  dirt,  oil,  and  copper,  or  carbon  dust 

armature. 

often  result  in  a  short-circuit. 

Short-circuit  oi 

switch. 

Runs  backward. 

Connect  up  correctly  per  diagram;  if  no  diagram  is 

Wrong  connec- 

at hand,  reverse  connections  to  brushes  or  others 

tions. 

until  direction  of  rotation  is  satisfactory. 

Note  from  Line  50.— 45,  Engine  fails  to  regulate.     46,   Overload.     47,  Short-circuit  in 
armature.     48,  Striking  or  rubbing  of  armature.     49,  Friction.     50,  Weak  magnetic  field 


112 


ELECTRICAL  MACHINERY 


[ART.   165 


165.  Dkect-current  Generator  and  Motor  Troubles— Continued 


Reversed      current 

A.    Use  current  from  another   machine  or  a  battery 

§ 

through  field  coils. 

through  field  in  proper  direction  to  correct  fault. 

•J 

Test  polarity  with  a  compass. 

a 

H 

Reversed     connec- 

B.   If  connections  or  winding  are  not  known,  try  one 

tions. 

way  and  test;  if  not  correct  reverse  connections,  try 

M 

^-< 

again  and  test. 

§ 

Earth's  magnetism. 

C.    Connect  up  per  diagram  for  desired  rotation,  see 

12 
'« 

that    connections    to    shunt    and    series    coils    are 

M 

properly  made.     See  57. 

1 

Proximity    of    an- 

D.   Shift  brushes  until  they  operate  better.     See  1,  2,  3. 

§ 

other  dynamo. 

> 
£ 

Brushes  not  in  right 

PS 

position.     See     1, 

2,3. 

Too  weak  residual  mag- 

Same as  58  A. 

netism. 

K 

Short-circuit  in  machine. 

See  12,  54,  56. 

! 

Short-circuit  in  external 

A    lamp    socket,    etc.,    may    be    short-circuited    or 

4> 

fl 

circuit. 

grounded,  and  prevent  building  up  shunt  or  com- 

& 

pound  machines.     Find  and  remedy  before  closing 

S 

switch.     See  54,  56. 

| 

Field    coils    opposed    to 

Reverse  connections  of  one  of  field  coils  and  test. 

1 

each  other. 

Find  polarity  with  compass;  if  necessary  try  58  A, 

« 

C,  D.     If  necessary  reverse  connections  and  recharge 

in  opposite  directions. 

Broken  wire. 

A.   Search  out  and  repair.     See  13. 

Faulty  connections. 

B.    Search  out  and  repair.     See  37. 

49 
'3 

Brushes      not      in 

C.    Search  out  and  repair.     See  5. 

1 

contact. 

o 

Safety  fuses  melted 

D.   Search  out  and  repair.     See  53  A. 

1 

or  broken. 

0 

Switch  open. 

E.    Search  out  and  repair.     See  53  D. 

External        circuit 

F.    Search  out  and  repair   with  dynamo  switch   open 

open. 

until  repairs  are  completed. 

Too      great      load      on 

Reduce  load  to  pilot  lamp  on  shunt  and  incandescent 

dynamo. 

machines;  after  voltage  is  obtained  close  switches 

in  succession  slowly,  and  regulate  voltage.     See  11 

A  and  65. 

Too  great  resistance  in 

Bring  up  to  voltage  gradually  with  rheostat,  and 

field  rheostat. 

watch  pilot  lamp;  regulate  carefully. 

SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     113 

166.  Process  of  Commutation. — The  path  of  the  current  is 
as  shown  in  Fig.  118,  wherein  A  is  the  carbon  brush;  C,  C",  C" 
are  the  commutator  segments;  B,  B',  B"  are  the  windings  of 
the  armature.     At  the  position  shown,  coil   B   is   short-cir- 
cuited by  the  brush,   the  current  passing  into  the  face  of 
the  brush  and  out  again  as  shown  by  the  dotted  line.     This 
local  current  may  be  many  times  greater  than  the  normal  in- 
tensity and  is  the  one  which  causes  pitting.     With  perfect 
commutation,   with   no   sparking   or   glowing,    there   should 
be  created  in  the  short-circuited  coil  under  the  brush,   by 
virtue  of  the  flux  from  that  pole-tip  away  from  which  the 
armature  is  revolving,  an  electromotive  force.     This  should 
be  just  great  enough  to  reverse  the  current  within  the  short- 
circuited    coil    and    render    it 

.  .Carbon  Brush 

equal  to  the  current  in  the  , 

Commutator 

winding  proper.     Since  on  one 

side  of  the  brush  the  current 

is  in  one  direction  and  on  the 

other  side  in  the  other  direc- 

tion,  the  act  of  commutation 

beneath  the  brush  reverses  this  FlG-   118. — Armature   coil  short- 

current  and  permits  it  to  in-      cireuited  when  c°mmutatil* 

crease  to  the  correct  intensity  in  the  opposite  direction. 

167.  With  Copper  Brushes  This    Reversal    of    Current 
Must  be  Very  Accurately  Effected. — With  carbon  brushes 
there  is  a  much  smaller  tendency  to  spark,  hence  they  will 
permit  of  a  certain  inexactness  of  commutation  adjustment. 
Experiments  indicate  that  the  carbon   can   resist   as  much 
as  a  3-volt  pressure  creating  current  in  the  wrong  direction 
and  still  not  spark  or  glow.     This  is  the  property  which  has 
necessitated  the  use  of  carbon  brushes  instead  of  copper  on 
most  apparatus.     When,  however,  this  voltage,  induced  in 
the  wrong  direction,  rises  above  3  volts  during  the  passage 
of  the  armature  coil  underneath  the   brush,   trouble  from 
sparking   and  glowing  occurs. 

This  is  the  reason  that,  in  a  motor,  the  brushes  are  pulled 
backward  as  far  as  possible  at  no-load,  so  that  the  coil 
short-circuited  by  the  brush  may  enter  the  fringe  or  flux 

8 


114  ELECTRICAL  MACHINERY  [ART.  168 

from  the  pole-tip,  thus  creating  the  proper  reversal  of  current 
during  the  time  the  coil  is  passing  under  the  brush.  Since 
adjacent  poles  are  opposite  in  polarity,  only  one  can  provide 
the  proper  flux  direction  for  this  reversal.  In  a  motor  it  is 
always  the  pole  behind  the  brush  and  thus  the  brush  requires 
a  backward  lead.  In  a  generator  it  is  the  pole  ahead  of  the 
brush  in  the  direction  of  rotation.  Hence,  generators  require 
a  forward  lead. 

168.  If  the  Motor  Gives  Trouble  from  Glowing  and  Pitting, 
the  cause  is  probably  this  induced  current,  and  the  remedy  is, 
first,  to  insure  that  the  lead  of  the  brushes  brings  them  in  the 
most  satisfactory  position.     If  no  lead  or  brush  position  can 
be  found  which  will  eliminate  the  trouble,  the  width  of  the 
brush  must  be  changed.     The  wider  the  brush  the  longer  does 
the  coil  suffer  short-circuit,    as   described.     Conversely    the 
narrower  the  brush,  the  quicker  must  the  current  be  reversed. 
There  is,  therefore,  a  width  of  brush  which   best  satisfies 
both  conditions.     Usually,   however,  where  glowing  occurs, 
the  cause  is  too  wide  a  brush,  and  often  serious  trouble  from 
this  cause  can  be  entirely  eliminated  by  varying  the  width  of 
the  brush  perhaps  only  %  in.;  see  Art.  196  on  "Glowing." 
Sparking  may  be  due  to  an  open  armature  circuit;  see  Art. 
200  for  a  description  of  the  symptoms  and  the  remedy. 

169.  Sparking  Due  to  Rough  Commutator. — The  commu- 
tator surface  may  not  be  perfectly  smooth  after  receiving  its 
last  "turn  off."     The  work  may  have  been  poorly  done  by 
the  manufacturer,  with  the  result  that  the  commutator  surface, 
instead  of  being  smooth,   is   somewhat  rough.     The   result 
(especially  with  high-speed  commutators)  is  that  the  brush 
does  not  make  good  contact  with  the  commutator  surface. 
It  may  chatter  and  thus  with  many  motors  (especially  those 
of  high  voltage)  the  operation  will  be  attended  with  sparking. 
As  a  result,  the  commutator  surface,  instead  of  becoming 
bright  and  smooth  with  time,  becomes  rough  and  dull  or  raw 
in  appearance.     Under  these  conditions  the  brushes  do  not 
make  good  contact,  and,  hence,  the  heat  generated,  even  under 
proper  commutator  conditions,  owing  to  the  resistance  of  brush 
contact,  is  multiplied  several  times,  with  consequent  increase 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     115 


of  temperature  of  the  commutator.  In  addition,  the  friction 
of  brush  contact  (which  should  give  a  coefficient  of  0.2)  is, 
with  a  rough  commutator,  much  higher  than  it  should  be. 
which  tends  to  increase  the  temperature. 

170.  Heating  of  Commutator*  may  develop  from  any  of  the 
following  causes;  (a)  Overload;  (6)  sparking  at  the  brushes;  (c) 
too  high  brush  pressure ;  (d)  lack  of  lubrication  on  commutator. 

171.  Hot   Commutator. — All  this   (see  above)   trouble  is 
cumulative.     The  result  is  that  finally  the  temperature  will 
rise  to  a  degree^where  the  solder  in  the  commutator  will  melt, 
perhaps  short-circuiting  or  open-circuiting  the  winding.     A 
commutator  will  stand  very  slight  sparking,  but  where  it  is 
noticeable  and  where  it  is  continued  for  long  periods  of  time, 


Armature 
CoiL 


Commutator 
Bar 


Clomping 

-" 


FIG.  119. — Section  of  direct-current  motor  armature. 

trouble  is  liable  to  result.  Where  the  load  is  usually  very  light 
and  where  full-load  or  overload  are  infrequent,  a  smoothing 
of  the  commutator  automatically  occurs  during  the  light-load 
period.  This  is  the  reason  that  certain  railway  motors,  which 
sometimes  show  sparking  under  their  normal  hour  rating  load, 
give  satisfaction  as  to  commutation.  The  coasting  of  the  car 
smooths  up  the  imperceptible  damage  done  by  the  sparking 
during  the  heavy  load. 

172.  Loose  Commutator  Segments. — A  further  and  more 
serious  cause  of  sparking  and  commutator  trouble  is  due  to  the 
fact  that  the  commutator  may  not  be  " settled"  when  shipped 
by  the  manufacturer.  A  commutator  is  made  of  many  parts 
(Fig.  119),  insulated  one  from  another,  and  all  bound  together 
by  mechanical  clamping  arrangements.  The  segments  them- 

*  WESTINGHOUSE  INSTRUCTION  BOOK. 


116  ELECTRICAL  MACHINERY  [ART.  173 

selves  are  held  by  a  clamp-ring  on  each  end,  which  must  be 
insulated  from  them  and  which  should  hold  each  segment  in- 
dividually from  any  movement  relative  to  another.  Since  the 
clamp  must  touch  and  hold  down  all  segments,  a  failure  to  do 
so  in  any  case  results  in  a  loose  bar,  which  moves  relatively 
to  the  next  bar  and  causes  roughness  and  thus  sparking,  with 
all  its  attendant  accumulative  troubles.  The  roughness  of 
commutators  due  to  poor  turning  or  to  poor  design  is  shown 
uniformly  over  all  the  surface  of  the  commutator  on  which 
brushes  rest.  A  roughness  due  to  a  high  or  loose  bar  is  shown 
by  local  trouble  near  the  bad  bar  and  its  corresponding  bars 
around  the  commutator.  The  jump  of  the  brush  occurs  at 
the  high  bar  and  is  the  cause  of  the  sparking.  See  also  Arts. 
174  and  178. 

173.  Blackening  of  the  Commutator  — Sparking  due  to  a 
loose  or  high  bar  causes  a  local  blackening  instead  of  a  uniform 
blackening,  which  occurs  in  case  of  poor  design  or  poor  com- 
mutator surface  resulting  from  poor  turning.     Also,  if  the  speed 
of  the  commutator  is  low  enough,  there  will  be  a  spark  at  the 
time  the  bad  segment  passes  the  brush.     At  ordinary  speeds, 
or  where  there  are  several  loose  bars,  the  sparking  in  appear- 
ance will  not  be  different  from  that  due  to  poor  design  or  poor 
turning.     In  such  a  case  an  examination  of  the  commutator 
surface  must  be  made  to  identify  the  cause.     The  slightest 
movement  of  a  bar,  especially  with  the  higher-voltage  and 
high-commutator-speed  machines,  may  cause  the  trouble.     A 
splendidly  designed  motor  may  show  very  poor  operation,  due 
to  a  commutator  fault. 

174.  Correcting  Commutator  Roughness. — The  proper  way 
to  correct  the  rough  surface  of  a  commutator  will  be  deter- 
mined by  the  condition  of  the  commutator.     Where  the  com- 
mutator is  very  rough  or  eccentric  the  armature  should  be 
taken  out  of  the  machine  and  the  commutator  turned  to  a 
true  cylindrical  surface  in  a  lathe.     Where  the  commutator  is 
only  reasonably  rough  it  may  be  trued  by  filing,  using  an  ar- 
rangement similar  to  that  in  Fig.  120,  which  is  described  in 
detail  in  a  following  paragraph.     Where  the  roughness  is  due 
to  poor  turning  or  to  ordinary  operation,  it  may  then  be 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     117 


smoothed  with  a  piece  of  ordinary  grindstone  (Fig.  121).     Sand- 
paper held  on  a  block  of  wood  (Fig.  122)  which  has  been  cut 


•Filing  Plane 


£  of  Brush — >•   k  -<tof  Shaft 

.Carbon 
Brush 

'••*&*  '?ML 

I   ^r-rrrV 

.--'Commutator-* 


Sheet  Metal 
Oao/e-—'. 


& 


I- Filing  Commutator  H-Use  of  Brush-Setting  Gage, 

FIG.  120. — Equipment    for    filing    commutators    and    fitting   brushes. 
(POWER,  Aug.  29,  1916,  p.  316.) 

to  fit  the  cylindrical  surface  of  the  commutator  may  also  be 
used  for  this  purpose  but  it  is 
not,  apparently,  as  effective  as 
the  grindstone. 

175.  To  Smooth  a  Commuta- 
tor with  a  Piece  of  Grindstone 
(Fig.  121)  it  should  be  cut  to 
convenient  size  and  held  by  the 
hand  against  the  commutator. 
If  possible,  it  should  be  rounded 

.•Piece  of  Grindstone 

:--"'Bindinq 
-*        W/ns 


Shaft- 


Direction  of-— 
'-•Armature  Coils  .  Rotation  of  Cornmv 

FIG.  121.— ^Smoothing  commutator    FIG.  122. — Sandpapering  blocks 
with  grindstone.  of  two  different  types. 

out  to  the  shape  of  the  commutator,  though  the  rounding  is 
not  absolutely  necessary  except  when  the  surface  is  exceed- 


118  ELECTRICAL  MACHINERY  [ART.  176 

ingly  irregular.  A  commutator  can  thus  be  ground  on  low- 
voltage  machines  without  removing  the  brushes  from  the 
commutator  and  during  the  ordinary  operation  of  the  motor 
under  load.  When  sparking  is  due  to  poor  turning,  grinding 
causes  the  sparking  to  entirely  disappear.  This  is  also  a  good 
method  of  cleaning  the  surface  of  brushes  which  have  become 
coated  with  copper  from  the  use  of  sandpaper  in  fitting  them 
to  the  commutator  surface. 

176.  Some  Kinds  of  Sandpaper  if  used  to    give  a    brush 
surface  or  to  smooth  a  commutator  with  the  brushes  down, 
imbed  in  the  face  of  the  brush  hard  material  which  lodges 
there,    cutting   the  commutator  and  thus    collecting  about 
itself  copper  from  the  commutator.    An  examination  of  the 
face  of  the  brush  after  running  a  time  will  show  these  col- 
lections either  in  spots  or  all  over  the  face  of  the  brush. 
The  sandstone,  used  as  suggested,  removes  all  this. 

177.  Where  Roughness  or  Sparking  are  Due  to  a  Loose 
Bar,  Grinding  Will  Do  No  Good. — Then  a  different  process 
for  correction  must  be  used.     It  consists  first  in  tightening 
the  clamp-rings  which  hold  down  the  segments  so  that  they 
touch  and  hold,  each  one  preventing    any    relative    move- 
ments   of  the  bars.     After  this  has  been   done,  produce  a 
smooth  commutator  surface  by  turning  if  the  bar  is  much 
displaced,  or  by  filing  or  grinding  if  it  is  but  slightly  dis- 
placed. 

178.  In  Truing  a  Commutator  with  a  File  (Fig.  120)  a  gage 
or  rest  of  some  sort  should  be  employed  to  insure  that  the  com- 
mutator will  be  improved  rather  than  injured  by  the  filing 
process.     The  file   rest   should   be  bolted  securely    to    the 
bearing  or  to  some  part  of  the  frame  of  the  machine.     The 
two  planes  of  the  rest  which  project  over  the  commutator 
should    lie  exactly   parallel   to   it.     Then    the    armature    is 
rotated  at  normal  speed  and  a  flat  smooth  file  is  carefully 
pushed  over  the  planes.     It  will  cut  away  the  metal  of  the 
planes    and    also   that   of   the    commutator.     Chalk  should 
be  used  on   the  file  to  prevent  it  from  tearing  the   copper 
and    the  file  should  be   cleaned  frequently.     At    intervals, 
place  a  steel  straight  edge  along   the  commutator  surface 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     119 


-^•flat-Head 'Adjusting Screws 
-Renewable  Filing  Plate , 


parallel  to  the  shaft  to  check  the  work  as  it  proceeds. 
After  filing,  the  commutator  surface  should  be  finished 
with  very  fine  sandpaper  or  preferably  with  a  piece  of 
grindstone  (Fig.  121)  as  described  above.  A  file  rest  of  a 
type  having  adjustable  filing-surface  plates  and  arranged 
for  bolting  to  the  pedestal  bearing  of  a  machine  is  illustrated 
in  Fig.  123. 

179.  To  Correct  Loose-commutator  Troubles. — First,  force 
the  clamps  of  the  commutator  down  firm,  so  that  when  the 
commutator  is  at  normal  temperature   the  clamping  rings 
cannot  be  screwed  down    further  without  excessive  effort. 
This  is  necessary  so  that  all  the  bars  may  have  a  direct 
pressure  from  the  clamp,  rendering  any  movement,   up  or 
down,  impossible.     Second,  after  having  drawn  the  clamps 
down,  smooth  off  the  surface  of 

the  commutator,  using   one  of 
the  processes  described  above. 

179a.  To  Get  the  Clamps 
Down  Firm  run  the  machine 
under  load;  if  roughness  ap- 
pears, shut  down  at  a  convenient 

f.  ,M      ,     ,      ,.  ,  ,  FIG.  123. — Showing  how  a  file 

time,    and,    while    hot,    tighten    rest  may  be  easily  constructed. 

the    clamping    rings.     If    it    is 

found  that  the  tightening  bolts  can  be  screwed  up  somewhat, 
the  machine  should  again  be  put  in  service  for  at  least  four 
hours  at  the  end  of  which  time  shut  it  down  again  and  again 
tighten  the  bolts.  If,  now,  no  more  slack  can  be  taken  up 
on  tightening  bolts,  the  commutator  should  be  surfaced,  either 
by  turning  with  a  tool  or  by  grinding. 

180.  The  Slotting  of  Commutators.*— There  seems  to   be 
a  prevalent  idea  that  slotting  should  cure   all    commutator 
troubles,    irrespective   of   their    causes.     This   is   not   true, 
but  slotting  is  a  cure  for  certain  specific  troubles.     Where 
the  peripheral  speed  of  the  commutator  is  so  slow  that  the 
dirt  which  may  collect  in  the  slots  between  commutator  bars 
will    not   be  thrown  out  by  centrifugal  force,   slotting  may 


*  Alan  Bennett,  AMERICAN  MACHINIST,  Sept.  26,  1912. 


120  ELECTRICAL  MACHINERY  [ART.  181 

aggravate  rather  than  correct  commutation  difficulties.     See 
also  Art.  186. 

181.  The   Principal   Reason  for  Slotting  Commutators  is 
to  relieve  the  commutators  of  high  mica,  that  is,  mica  that 
projects  above  the  surface.     High  mica  is  generally  due  to 
one  of  two  causes :  either  the  mica  is  too  hard  and  does  not 
wear  down  at  an   equal  rate  with  the  copper,  or  the  com- 
mutator  does   not   hold   the    mica    securely    between    the 
segments,  allowing  it  to  work  out  by  the  combined  action 
of  centrifugal  force  and  the  heating  and  cooling  of  the  com- 
mutator.    It  is  evident  that  a  commutator  with  a  surface 
made  irregular  by  projecting  mica  rotating   at   high   speed 
under  a  brush,  must  impart  to  the  brush  a  vibratory  move- 
ment, and  thus  impair  the  close  contact  that  should  exist 
between    the  brush  and  commutator.     The   result   is  that 
sparking  takes  place  more  or  less  violently,  depending  on 
the   condition  of  the  commutator  surface  and  the  rate   of 
speed.     This  condition  generally  manifests   itself   after   the 
machine  has  been  running  for  some  time,  and  in  many  cases 
will  account  for  the  development  of  sparking  which  did  not 
occur  at  the  time  of  installation.     Often  a  case  of  this  kind 
is  aggravated  by  increasing  the  brush  tension,  causing  a  still 
faster  rate  of  wear  of  copper  over  mica,  with  an  attendant  in- 
creased heating  of  the  commutator. 

182.  What  is  Accomplished  by  Slotting. — A  harder  brush 
may  at  times  be  used,  with  the  idea  of  grinding  off  the  mica 
and  thus  bringing  it  down  to  the  commutator  surface.     Instead 
of  curing  the  trouble,  the  commutator  will,  in  the  majority  of 
cases,  assume  the   raw  appearance  of   being   freshly  sand- 
papered, instead  of  the  glossy  surface  it  should  have,  and 
both  brush  and   commutator  will  wear  rapidly.     This   con- 
dition can  be  restored  to  normal  and  the  commutator  kept 
to  a  true  surface  by  slotting,  after  which,  with  proper  care 
and   the   use   of  proper   brushes,  commutator  troubles  will 
generally  cease,  provided  the  electrical  design  of  the  machine 
is  not  at  fault.      Even  then  there  are  cases  that  may  be 
benefited  to  a  certain  extent  by  slotting,  by  reason  of  the 
good  brush  contact  obtained.     The  majority  of  cases  that 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     121 

show  improvement  are  the  ones  in  which  the  trouble  is  not 
inherent  in  the  design  of  the  machine,  but  is  due  to  mechanical 
causes. 

183.  With  a  Slotted  Commutator  It  is  Possible  to  Use  a 
Brush  of  Fine  Grain  and  Soft  Texture,  inasmuch  as  there  is 
not  the  same  tendency  to  wear  away  the  brush  as  with  an 
unslotted  commutator.     The  commutator  should  then  take 
on  the  much-desired  polish  that  is  generally  not  attainable 
with  the  harder  brush.     The  life  of  both  brush   and  com- 
mutator will  be  increased,  and  friction  and  the  consequent 
heating   will  be   reduced.     These  advantages   will   effect   a 
saving  that  will  more  than  offset  the  cost  of  slotting. 

184.  Various  Methods  of  Slotting. — There  is  a  variety  of 
slotting  devices  on  the  market.    Some  are  designed  to  operate 
with   the  armature  swung  between  the   centers  of  a  lathe; 
others    use    a    special    tool  in  a  shaper,  with  the  armature 
secured  to  its  bed.     Still  others  are  used  by  hand  with  the 
armature  resting  on  blocks.     In  all  cases  the  full  width  of  the 
mica  should  be   removed,   and   the   resulting  slot  carefully 
cleaned  from  burrs  and  rough  edges.     It  is  not  necessary  that 
the  slotting  be  carried  deeply   in   the    commutator.     One- 
sixteenth  of  an  inch  is  generally  considered  sufficient.     See 
also  Art.  186. 

185.  A  Slotted  Commutator  Should  Have  Proper  and  Fre- 
quent Care,  as  there  is  a  chance  of  small  particles  of  copper 
being  dragged  across  from  bar  to  bar,  and  for  dirt,  oil  and 
carbon  dust  to  accumulate  in  the  slots  and  short-circuit  the 
commutator. 

186.  High  Mica  in    Commutators. — Some    direct-current 
generators  and  motors,  under  certain  conditions,    roughen 
up  their  commutators  after  a  short  term  of  service,  although 
there   seems  to  be  no  excessive  sparking  under  or  at  the 
edges  of  the  brushes.     This   may   occur   even   though   the 
commutator    has    been   well    "settled."     The    commutator 
performs  as  if  the  mica,  used  between  bars  to  insulate  the 
various  segments  one  from  another,  had  protruded  upward, 
causing  roughness  and  excessive  sparking. 


122  ELECTRICAL  MACHINERY  [ART.  187 

187.  Actual  Raising  of  the  Mica  is  a  Very  Rare  Occur- 
rence, and,  if    it  occurs,  does   so  at   certain  spots  and  is 
easily  and    positively   identified.     An    actual  uniform    pro- 
truding of  mica,  all  over   a  commutator,    as   described,  is 
practically  an  unknown    phenomenon.     What  actually  does 
occur  is  an  eating  away  of  the  copper  surface  of  the  com- 
mutator, leaving  the  high  mica  between  the  bars.     A  good 
machine  will  not  spark  enough  to  cause  this  condition.     A 
poor  machine  will. 

188.  The  Phenomenon  of  High  Mica  is  Easily  Identified, 
as  the  commutator  surface  appears  "raw"  all   over  instead 
of  smooth  and  bright  with  a  good  brown  gloss.     If  allowed 
to  continue,  a  general  roughness  appears,  accompanied  by 
sparking,  until  finally  the  sparking  and  heating  will  increase 
so  much  that  the   machine  may  flash  over  from  brush  to 
brush,  blowing  the  fuses   or    opening    the    circuit-breakers. 
The  trouble  is  aggravated  if  the  motor  operates  continuously 
under  heavy  load.     If  there  are  periods  of  light  load,  the 
commutator  has  an   opportunity  to  be  smoothed  down  by 
the  brushes.     This  condition  is  appreciated  by  railway  motor 
designers.     A  railway   motor  coasts  a  considerable  portion 
of  the  time.     Thus  the  commutator  is  smoothed,  neutral- 
izing the  roughening  occurring  under  load. 

189.  To  Remedy  a  Roughened,  High-mica  Commutator. 
— (1)  Use  the  machine  on  work  where   the  load  is  some- 
what intermittent;   (2)  replace  it  altogether;  or  (3)  slot  the 
commutator.     Then,  as  there  are   no   longer  two   different 
materials  to  wear  down  or  to  be  worn  away  by  sparking,  an 
unequal    surface    will    not    result.      The  mica  need  be  cut 
down  only  }{Q  in.  and  a  narrow,  sharp  chisel   will  do  the 
work    satisfactorily.     No   trouble   will    result    from    short- 
circuiting  in  this  case  (if  the  rotational  speed  of  the  machine 
is  sufficiently  great)  since  centrifugal  force  keeps  the  slots 
clean.     Some  manufacturers   ship   their  machines  with  the 
commutators  slotted. 

190.  Brushes,  Their  Adjustment  and  Care.* — The  position 
of  the  brushes  on  a  direct-current  machine  should  be  on  or 

*  WESTINQHOUSE  INSTRUCTION  BOOK. 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     123 


near  the  no-load  neutral  point  of  the  commutator.  This 
neutral  point  on  most  standard,  non-commutating-pole  ma- 
chines is  in  line  with  the  center  of  the  pole  and  the  brushes 
should  be  set  a  little  in  advance  of  this  neutral  point.  The 
brushes  of  non-commutating-pole  generators  should  be  given 
a  slight  " forward  lead"  in  the  direction  of  rotation  of  the 
armature.  Motor  brushes  should  be  set  somewhat  back  of 
the  neutral  point,  the  "backward  lead"  in  this  case  being 
approximately  equal  to  the  forward  lead  on  generators.  The 
exact  position  in  either  case  is  that  which  gives  the  best 

commutation  at  normal  voltage 
for  all  loads.  In  no  case  should 
the  brushes  be  set  far  enough 
from  the  neutral  point  to  cause 
dangerous  sparking  at  no-load. 


Brush 

BrushHo/der 


Sand 


FIG.  124. — Sandpaper  should  be 
held  close  to  the  commutator. 


FIG.  125. — Sandpapering 
brushes. 


191.  The  Ends  of  All  Brushes  Should  be  Fitted  to  the 
Commutator  so  that  they  make  good  contact  over  their  en- 
tire bearing  faces.  This  can  be  most  easily  accomplished 
after  the  brush  holders  have  been  adjusted  and  the  brushes 
inserted.  Lift  a  set  of  brushes  sufficiently  to  permit  a  sheet 
of  sandpaper  to  be  inserted.  Draw  the  sandpaper  under  the 
brushes  (Figs.  124  and  125)  being  careful  to  keep  the  ends  of 
the  paper  as  close  to  the  commutator  surface  as  possible  to 
avoid  rounding  the  edges  of  the  brushes.  Start  with  coarse 
sandpaper  and  finish  with  fine  sandpaper.  The  sandpaper 
should  be  pulled  under  the  brushes  only  in  the  direction 
of  rotation  of  the  commutator  because,  since  brushes  always 
fit  loosely  in  their  holders,  accurate  fitting  is  otherwise  impos- 


124 


ELECTRICAL  MACHINERY 


[ART.  192 


sible.  When  pulling  the  strip  of  sandpaper  back  to  the  start- 
ing position  the  brush  should  be  raised  in  its  holder  so  that  it 
does  not  then  touch  the  sandpaper.  Each  set  of  brushes 
should  be  similarly  treated  in  turn.  If  the  brushes  are  copper- 
plated,  their  edges  should  be  slightly  beveled,  so  that  the 
copper  does  not  contact  with  the  commutator. 

192.  A  Carbon  Brush  Should  Exert  a  Pressure  of  about 
Ib.  per  sq.   in.   on   the   commutator.     The  pressure  in 

any  case  may  be  determined  by 
using  a  spring  balance  as  sug- 
gested in  Fig.  126.  Then,  the 
tension  of  the  spring  should  be 
adjusted  so  that:  Total  pressure 
in  pounds  -f-  brush  contact  area 
in  square  inches  =  1%.  The 
brush  pressure  is  the  reading,  in 
pounds,  of  the  spring  balance 
when  the  tension  exerted  is  just 
sufficient  to  raise  the  brush 
from  the  surface  of  the  commu- 
tator. 

193.  Brush  Contact  Resist- 
ance.—  If  adjustment  of  brush 
position  gives  no  relief,  the  fit 
of  the  brush  upon  the  commu- 

'••Commutator  tat°r    "^    be     P001'          AllV     r6- 

FIG.  126.— Method  of  deter-  sistance  in  series  with  a  motor 
mining  pounds  brush  pressure  armature  causes  a  drop  in  speed 
with  a  spring  balance.  ,  i  ,  -, 

as  the  load  comes  on,  the  drop 

increasing  almost  directly  in  proportion  to  the  armature- 
circuit  resistance  (see  Art.  58). 

The  resistance  of  surface  contact  of  a  carbon  brush  is  a 
formidable  factor,  particularly  with  low-voltage  and  high-cur- 
rent machines.  This  contact  resistance  is,  at  ordinary  brush 
densities,  about  0.028  ohm  per  sq.  in.  The  specific  resistance 
of  carbon  itself  is  0.002  ohm.  Hence,  for  ordinary  carbon,  the 
contact  resistance  only  is  of  consequence.  The  resistance  of 
the  carbon  is  negligible. 


SEC.  4]   DIRECT-CURRENT  GENERATORS  AND  MOTORS      125 

EXAMPLE. — An  ordinary  carbon  brush  operates  at  a  current  density 
of  35  amp.  per  sq.  in.  Then  assuming  a  machine  having  one  brush  of 
1  sq.  in.  cross-section  for  each  polarity  the  brush  contact  voltage  drop 
for  the  machine  is  2  X  35  X  0.028  =  1.96  volts.  The  specific  resistance 
is  0.002  ohm,  giving  a  voltage  drop  through  the  carbon  itself,  assuming 
a  length  of  brush  of  1%  m->  of  0.21  volt,  which  is  negligible  as  compared 
with  the  drop  of  2  volts  due  to  surface  contact. 

194.  The  Drop  in  Voltage  Due  to  Contact  Resistance  is 
Greatly  Increased  if  the  Fit  of  the  Brushes  or  the  Commutator 
is  Poor,  which  may  be  due  either  to  the  cutting  of  grooves  in 
its  face  by  the  use  of  coarse  sandpaper,  or  to  a  part  of  the 
carbon  not  touching  the  commutator.     Hence,  when  the  drop 
of  speed  is  excessive,  this  condition  should  be  carefully  in- 
spected.    Finally,  the  spacing  of  the  brushes  must  be  checked. 
Unequal  spacing  may  not  only  produce  a  large  drop  in  speed, 
but  will  reduce  the  efficiency  and  life  of  both  brushes  and 
commutator. 

195.  Sparking  of  the  Brushes  may  be  due  to  one  of  the 
following  causes*  (see  also  Dynamo-troubles  Table) :  (a)  The 
machine  may  be  overloaded;  (6)  the  brushes  may  not  be  set 
exactly  at  the  point  of  commutation — a  position  can  always 
be  found  where  there  is  no  perceptible  sparking,  and  at  this 
point  the  brushes  should  be  set  and  secured;  (c)  the  brushes 
may  be  wedged  in  the  holders;  (d)  the  brushes  may  not  be 
fitted  to  the  circumference  of  the  commutator;  (e)  the  brushes 
may  not  bear  on  the  commutator  with  sufficient  pressure;  (/) 
the  brushes  may  be  burnt  on  the  ends;  (g)  the  commutator 
may  be  rough;  if  so,  it  should  be  smoothed  off;  (h)  a  com- 
mutator bar  may  be  loose  or  may  project  above  the  others; 
(i)  the  commutator  may  be  dirty,  oily  or  worn  out;  (j)  the 
carbon  in  the  brushes  may  be  unsuitable ;  (k)  the  brushes  may 
not  be  equally  spaced  around  the  periphery  of  the  commuta- 
tor; (I)  some  brushes  may  have  extra  pressure  and  may  be 
taking  more  than  their  share  of  the  current;  (ra)  high  mica; 
(ri)  vibration  of  the  brushes.    The  above  are  the  more  com- 
mon causes,  but  sparking  may  be  due  to  an  open  circuit  or 
loose  connection  in  the  armature  (see  Art.  200). 

*  WESTINQHOUSE  INSTRUCTION  BOOK. 


126  ELECTRICAL  MACHINERY  [ART.  196 

196.  Glowing  and  Pitting  of  Carbon  Brushes  may  be  due  to 
either  of  two  causes,  poor  design  or  a  wrong  position  of  the 
brushes  on  the  commutator.     The  error  of  design  may  be  only 
in  the  choice  of  width  of  carbon  brush  used.     The  pitting  is 
due  to  glowing.     If  the  glowing  is  at  the  edge  of  the  carbon  it 
is  plainly  visible  and  easily  located.     It  may,  however,  occur 
underneath  the  carbon  so  that  it  can  be  seen  only  with  diffi- 
culty.    Such  glowing  pits  the  carbon  face  by  heat  disintegra- 
tion.    With  some  machines  three-fourths  of  the  brush  face 
may  be  eaten  away  and  the  pits  may  be,  perhaps,  J^  in.  to  J^ 
in.  deep  when  discovered.     A  usual  (incorrect)  decision  is  that 
the  current  per  square  inch  of  contact  is  too  great,  the  calcu- 
lation being  made  by  dividing  the  line  amperes  by  the  square 
inch  cross-section  of  either  the  positive  or  the  negative  brushes. 
If  this  calculation  gives  a  value  under  45  or  50,  it  is  certain 
that  the  cause  of  the  trouble  has  not  been  judged  correctly. 

197.  The  Real  Cause  of  Glowing  is,  to  be  sure,  excessive 
current  through  the  brush,  but  this  is  not  the  line  current  if 
the  calculation,  as  stated,  shows  a  brush-face  density  below 
50  amp.  per  sq.  in.     It  is  a  local  current  caused  by  the  short- 
circuiting  of  two  or  more  segments  of  the  commutator  by  the 
brush  resting  upon  them.     The  usual  overlap  of  a  carbon  brush 
is  about  two  segments,   and  while  these  two  segments  are 
under  the  brush,  the  armature  coils  connected  to  them  are 
short-circuited.     If  the  design  of  the  machine  is  such  that  the 
coil  so  short-circuited  encloses  stray  flux  from  the  pole-tip, 
this  flux  will  create  in  the  short-circuited  coil  a  current,  per- 
haps many  times  larger  than  the  brush  is  capable  of  carrying, 
with  the  result  that  the  glowing  and  pitting  occurs. 

198.  Blow  Holes  in  Machine  Frame  Castings  Sometimes 
Cause  Sparking. — Such  blow  holes,  Fig.  127,  may  be  large 
enough  to  increase  the  reluctance  of  the  magnetic  circuit  suf- 
ficiently that  the  field  intensity  in  the  air  gap  is  not  sufficient 
to  eliminate  sparking.     It  has  been  proposed*  to  use  X-ray 
apparatus  to  locate  blow  holes  in  electrical  machine  castings 
when  the  indications  are  that  high  reluctance  in  the  magnetic 
circuit  is  the  cause  of  brush  difficulties. 

•  ELECTRICAL  WORLD,  May  1,  1915,  p.  1122. 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     127 


Frame 


199.  Chattering  of  Brushes  is  sometimes  experienced  on 
direct-current  machines.  Chattering  under  certain  conditions 
may  become  so  prominent  as  to  not  only  be  of  annoyance,  but 
as  to  actually  break  the  carbons.  An  examination  of  the  com- 
mutator will  reveal  no  rough- 
ness, the  surface  being,  perhaps, 
perfectly  smooth  and  bright. 
This  trouble  occurs  principally 
with  the  type  of  brush  holder 
which  has  a  box  guide  for  the  car- 
bon. The  spring  which  forces 
the  brush  into  contact  rests 
on  top  of  the  carbon  which  has 
fairly  free  play  in  the  box  guide. 
Chattering  usually  occurs  with 
high-speed  commutators,  run- 
ning at  4,000  to  5,000  ft.  per 
min.,  peripheral  speed. 

Such  brush  holders  are  necessary  for  commutators  which, 
like  those  on  engine-driven  machines,  may  run  out  of  true  on 
account  of  the  shaft  play  in  the  bearings  caused  by  the  recip- 
rocating motion  of  the  engine.  The  clamped  type  of  holder 
is  usually  free  from  bad  chattering  but  rocks  on  a  commuta- 


FIG.  127. — Showing  "blow 
holes"  in  a  direct-current  ma- 
chine frame. 


(Brush  Angle  1oo  Great)     (Running  in  the  Wrong  Direction) 
Orrect  Incorrect  Settings- 

FIG.  128. — Methods  of  setting  brushes. 

tor  that  runs  out,  causing  poor  contact  and  perhaps  sparking. 
Lubricating  the  commutator  causes  the  chattering  to  imme- 
diately disappear,  but  there  is  no  commutator  compound 
which  gives  a  lubricating  effect  lasting  over  possibly  a  half 
hour.  Thus  it  is  not  practical  to  lubricate  often  enough  to 


128  ELECTRICAL  MACHINERY  [ART.  200 

prevent  the  chattering.  There  will  be  no  chattering  if  the  angle 
of  the  brush  with  the  radial  line,  passing  through  the  center  of 
the  carbon  and  the  center  of  the  commutator,  is  less  than  10 
deg.  and  if  the  carbon  trails  on  the  commutator  instead  of 
leads.  Fig.  128,  7,  shows  the  setting  which  will  stop  all  serious 
chattering  and  Fig.  128,  77  and  777  show  settings  which  may 
give  trouble. 

200.  Sparking  may  be  Due  to  an  Open  Armature  Circuit. 
— The    "open"    may    be    in    the  winding  of  the  armature 
proper  or  it  may,  and  more  frequently,  be  due  to  a  loose  or 
broken  connection  where  the  armature  lead  is  soldered  to  a 
commutator    bar.     (Open    circuits    in    armatures    may    be 
located  by  testing  methods  as  described  in  Art.  213.)     Where 
an  armature  lead  is  thus  broken,  the  most  effective  remedy 
is  resoldering.     If  the  location  of  the  open  circuit  is  within 
the  winding  and  hence  not  readily  available,  the  commutator 
bars  on  each  side  of  the  "open"  can,  for  a  temporary  repair, 
be  shunted  by  bridging  a  piece  of  copper,  by  soldering  or  other- 
wise, across  the  two  segments  on  either  side  of  the  break. 

201.  The  Symptoms  of  Trouble  Due  to  an  Open  Armature 
Circuit  are  vicious  sparking  when  the  machine  is  in  operation. 
The  sparking  due  to  this  defect  is   unlike  that  due  to  any 
other  because  the  resulting  spark  is  "long"  and  " heavy"  and 
frequently  appears  to  extend  around  the  entire  circumfer- 
ence of  the  commutator.     A  bright   spark  will   occur  each 
time  the  break  passes  the  brush  position.     It  is  particularly 
destructive  in  its  action  and  shortly  "eats  away"  the  mica 
between  the  two  segments  on  each  side  of  the  open.     Thus, 
on  a  machine  which  has  been  in  operation,  the  offending  coil 
may  be  located  by  the  damaged  segments  to  which  it  con- 
nects.    The  temporary  bridge  referred  to  above  should  be  ar- 
ranged between  these  two  segments  and  around  the  "eaten- 
away"  mica. 

202.  Ring  Fire*  is  the  name  that  has  been  given  to  that 
type  of  sparking  where  rings  of  fire  embrace  the  circumference 
of  the  commutator,  wholly  or  partially  encircling  it.     Ring  fire 


p.  440. 


SEC.  4]   DIRECT-CURRENT  GENERATORS  AND  MOTORS     129 

may  be  subivided  into  two  classifications:  (1)  ordinary  ring 
fire  which  is  of  a  reddish  color  and  which  may  exist  to  a 
limited  extent  with  all  machines;  (2)  ring  fire  caused  by  arma- 
ture defects,  which  is  a  bluish-green  color  and  more  intense 
than  the  ordinary  type.  Ring  fire  is  ordinarily  due  to  minute 
arcs  between  adjacent  commutator  bars.  The  condition  may 
be  aggravated  by  conducting  materials  lodged  in  or  on  the 
surface  of  the  mica  insulation  between  bars.  Current  passing 
through  these  conducting  paths  between  the  bars  renders 
the  particles  incandescent.  The  fine  carbon  ground  from  the 
brushes  by  the  normal  operation  of  the  machine  or  particles  of 
copper  from  a  newly  turned  commutator  are  the  most  frequent 
causes  of  this  difficulty.  Secondary  causes  are  oil,  paraffin 
and  commutator  compounds  which  are  sometimes  used,  in 
which  particles  of  conducting  material  may  lodge.  Further- 
more, oil  may  carbonize  on  the  mica  segments,  forming  a 
conducting  path.  Where  mica  insulation  between  segments 
has  been  eaten  away  in  certain  isolated  locations  it  is  probable 
that  the  difficulty  is  due  to  the  carbonizing  of  oil  or  some  other 
materials  above  suggested. 

Undercut  commutators,  particularly  those  rotating  at  low 
peripheral  speed,  are  particularly  subject  to  ring  fire  because 
oils,  greases  and  conducting  materials  can  readily  lodge  be- 
tween segments  due  to  the  undercutting,  hence  the  com- 
mutators of  slow-speed  machines  should  be  cleaned  frequently 
with  a  stiff  brush  to  prevent  the  lodgment  of  foreign  materials 
in  them.  High-peripheral-speed  commutators  will  not,  ordi- 
narily, require  such  treatment,  because  with  them  the  centrif- 
ugal force  developed  is  usually  sufficient  to  prevent  the  re- 
tention of  the  dirt  in  the  slots  between  segments.  In  machines 
of  certain  designs  the  voltage  between  the  adjacent  segments 
under  the  pole-tips  may  be  great  enough  to  produce  ring  fire. 
The  compensated  winding  (Art.  37)  provides  an  effective 
correction  for  this  difficulty.  Where  the  mica  segments  are 
thin,  ring  fire  is  more  liable  to  occur.  Furthermore,  it  may  be 
encountered  more  frequently  with  slow-speed  than  with  high- 
speed machines  because  with  a  high-peripheral-speed  commu- 
tator the  segments  do  not  remain  in  the  zones  where  the  ring 
9 


130 


ELECTRICAL  MACHINERY 


[ART.  203 


fire  is  developed  for  a  sufficiently  long  period  to  permit  the 
formation  of  the  minute  arcs. 

203.  Flashing  is  that  sort  of  commutator  sparking  where 
an  arc  attains  considerable  length  and  flashes  between  brush 
holder  studs.  It  may  occur  in  a  normal  machine  at  the  in- 
stant when  an  excessively  high  e.m.f.  is  impressed  across  the 
machine  or  it  may  be  due  to  the  cumulative  effect  of  a  number 
of  the  causes  which  promote  sparking.  Flashing  is  more  liable 
to  occur  in  motors  than  in  generators. 


I- Connections 


E-Explorjng  Terminal 


FIG.  129. — Circuit  and  equipment  for  testing  direct-current  armatures 
for  trouble  with  direct-current. 

204.  Direct-current  Armatures  can  be  Tested  for  the  Com- 
mon Troubles  with  the  arrangement  of  Figs.  129  and  131. 
Terminals  6  and  c  are  clamped  to  the  commutator  at  opposite 
sides  and  connected  with  a  source  of  steady  current  through 
an  adjustable  resistance  and,  preferably  but  not  necessarily, 
an  ammeter.  The  terminals  of  a  low-reading  voltmeter  (a 
galvanometer  can  often  be  used)  are  connected  to  two  bare 


SEC.  4]   DIRECT-CURRENT  GENERATORS  AND  MOTORS     131 

metal  points  or  "  exploring  terminals/'  which  are  separated 
by  an  insulating  block,  j.     Exploring  terminals  may  also  be 


,  Rubber  Insulation 
•on  Sol  id 
\Conductor$ 


with  Sticks 
wood  Inside  to 

Bareol  Wire  Ends          Make  Handle 
^Filecf  to  Points  Stiff 


Ammeter 


FIG.  130.  FIG.  131. 

FIG.  130. — A  type  of  easily-made  exploring  terminals.  (The  flexible 
cord  leads  are  soldered  to  the  solid  copper  wires  within  the  tape-wrap- 
ping handle.) 

FIG.  131. — Method  of  testing  an  armature. 

arranged  as  shown  in  Figs.  129,  //  and  130.     In  use,  the 
current  is  adjusted  to  produce  a  convenient  deflection  of  the 


FIG.  132. — Illustrating  a  method  of  locating  faults  in  an  armature  with 
alternating  current. 

voltmeter  when  each  of  the  points  rests  on  an  adjacent  bar. 
The  points  are  moved  around  the  commutator  and  bridged 


132 


ELECTRICAL  MACHINERY 


[ART.  205 


across  the  insulation  between  every  two  bars.  If  the  voltmeter 
deflection  is  the  same  for  every  pair  of  bars  it  indicates  that 
there  is  no  short-circuit  in  the  armature. 

205.  In  Testing  Armatures  Where  Only  Alternating  Current 
or  Low-voltage  Cells  are  Available  a  telephone  receiver  (Figs. 
132, 133  and  134)  may  be  used  instead  of  an  electrical-measuring 


•10'  Twisted  Lamp  Cora 


Single 


'ing 
Lamp  Cord 
''Wooden  Handles 
#8  Steel  Wire 
Heed  I e  Points, 


FIG.  133. — Fault-locating  outfit  consisting  of  telephone  receiver,  vibrator 

and  dry  cells. 

instrument  for  a  detector.  The  variations  or  appearance  or 
disappearance  of  the  sound  in  the  receiver,  as  the  test  is  being 
conducted,  enables  the  operator  to  localize  the  trouble.  The 
alternating  current  (Fig.  132)  will  cause  a  hum  in  the  telephone 
receiver  and  the  pulsating  current  produced  by  the  vibrator 
(Figs.  133  and  134)  effects  the  same  result.  The  vibrator  may 


I-Locating  Short  Circuits  E-Locating  Grounds. 

FIG.  134. — Circuits  for  locating  faults  in  an  armature  with  a  low -voltage 
source  and  a  vibrator. 

be  an  ordinary  vibrating  bell  or  buzzer  connected  in  series  in 
the  circuit  as  shown  in  the  illustrations. 

206.  High- voltage  Alternating  Current  for  Testing  Arma- 
tures is  sometimes  used  (Fig.  135)  for  high-resistance  grounds 
which  may  not  be  detectable  where  a  low  e.m.f.  is  used  for 
testing.  Such  high-resistance  grounds  may  not  in  normal 
operation  make  their  presence  known  until  there  is  also  a 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     133 

ground  on  the  supply  system  'feeding  the  motor.  In  using 
this  apparatus  the  procedure  is  merely  to  connect  one 
terminal,  A,  of  the  high-voltage  source  to  the  commutator, 
and  the  other  terminal,  B,  to  the  shaft.  When  the  high 
voltage,  EH,  is  impressed  on  the  armature,  this  will,  usually, 
break  down  the  insulation  at  the  weak  point  and  render  the 
location  of  the  ground  apparent  by  the  smoke  which  will 
emanate  from  the  point  of  breakdown.  Fuses,  or  circuit- 
breakers,  Cit  <?2,  should  be  inserted  in  the  supply  circuit  to 
prevent  the  disastrous  effects  of  a  short-circuit. 

^'Step-up  Transformer 


FIG.  135. — Method   of   obtaining   and   applying   "high  potential"  for 
armature  fault  location. 

207.  An  Alternating-current  Inducing  Coil  is  Sometimes 
Used  for  Localizing  Armature  Troubles.* — Such  a  coil  is 
illustrated  in  Fig.  136  while  the  methods  of  its  application  are 
illustrated  in  Fig.  137.     The  inducer  is  merely  an  alternating- 
current  magnet  having  a  U-shaped  core  composed  of  sheet- 
steel  laminations.     The  laminations  should  be  interleaved  and 
hinged  at  the  point  H  so  that  the  core  may  be  opened  or 
closed  to  most  effectively  accommodate  armatures  of  different 
diameters.     In  use  the  core  should  be  opened  to  such  a  width 
that  its  two  poles,  PI  and  Pz,  will  have  a  separation  corre- 
sponding to  the  throw  of  one  armature  coil.     As  shown  in 
Fig.  137  the  poles  should  not  touch  the  armature  iron  when  a 
test  is  being  made,  but  an  air  gap  of  approximately  %  in. 
should  be  allowed. 

208.  The  Principle  of  Operation  of  the  Inducer  is  This.— 
The  alternating  flux  developed  by  the  current  in  the  inducer 
winding  sets  up  an  e.m.f.  in  the  armature  coils  and  if  a  coil 
is  short-circuited  it  will  cause  a  current  to  circulate  in  that 

'  PRACTICAL  ENGINEER,  Oct.  1,  1914,  p.  965. 


134 


ELECTRICAL  MACHINERY 


[ART.  208 


•Lifting  Hook 


FIG.  136. — "  Inducer"  used  in  locating  faults  in  a  direct-current  armature 
winding.     It  induces  an  alternating  e.m.f.  in  the  armature  winding. 


l-Loc*ting  a  Short  Circuit  Jt'Lcfeating  a  Ground 

FIG.  137. — Arrangements  for  locating  short  circuits  and  grounds  with 
the  alternating-current  inducer. 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     135 

coil.  Thus,  in  making  a  test  the  excited  inducer  is  held 
over  the  armature,  which  is  rotated.  Now,  if  the  arma- 
ture is  slowly  rotated,  while  a  small  piece  of  metal  is  held 
lightly  on  the  commutator  so  that  it  will  bridge  the  seg- 
ments between  each  of  the  bars,  there  will  be,  as  the  armature 
is  rotated,  an  arc  produced  each  time  one  of  the  segments  is 
bridged,  provided  the  segments  connect  to  a  normal  coil. 
But,  if  a  coil  is  open,  short-circuited  or  reversed,  there  will 
be  no  sparking.  To  determine  whether  the  difficulty  is 
due  to  an  open  or  a  short-circuit,  the  small  iron  test  strip 
is  .bridged  across  the  armature  slots.  When  the  coil  is 
short-circuited  the  induced  current  which  will  flow  in  it  will 
create  a  flux  around  the  sides  of  the  coil  which  can  be  de- 
tected with  an  iron  test  strip  which  is  moved  over  the  arma- 
ture coil  slots.  Furthermore,  a  short-circuited  coil  will  be 
quickly  heated  by  the  local  current  flowing  in  it. 

209.  The  Telephone  Receiver  Detector  Used  with  the 
Inducer,  Fig.  137,  affords  a  more  accurate  but  somewhat 
slower  method  than  that  described  in  preceding  Art.  207. 
To  locate  a  short-circuit,  Fig.  137,  7,  an  exploring  terminal 
connected  to  the  receiver  is  held  on  the  commutator  while 
the  armature  is  rotated.  When  the  exploring  points  are 
bridging  a  normal  coil  there  will  be  a  hum  in  the  receiver 
due  to  the  induced  alternating  current  flowing  through  it. 
When  the  points  bridge  a  short-circuited  coil  there  will  be 
little  or  no  hum  in  the  receiver,  depending  upon  the  number 
of  short-circuited  turns.  When  the  exploring-needle  points 
bridge  an  open  coil,  an  alternating  current  of  considerable 
intensity  will  flow  through  the  receiver  and  a  loud  click  will 
be  heard.  Where  a  coil  is  reversed,  several  of  the  coils  ad- 
jacent to  it  will  test  "silent."  To  definitely  locate  the  in- 
correctly connected  coil,  it  will  be  necessary  to  raise  some  of 
the  coils  and  trace  them  out  by  inspection.  To  locate  a 
ground,  the  connections  are  made  as  shown  in  Fig.  137,  //, 
and  the  armature  is  slowly  rotated.  When,  as  the  armature 
is  rotated,  contactor  C  is  on  the  segment  connecting  to  the 
grounded  coil,  no  sound  will  be  heard  in  the  receiver  but  at 
other  locations  there  will  be  a  sound,  the  sound  diminishing 


136 


ELECTRICAL  MACHINERY 


[ART.  210 


•CommuTaror 


in  intensity  as  the  segment  connecting  to  the  grounded  coil  is 
approached.     For  locating  partial  grounds,  this  method  may 

be  ineffective,  in  which  case 
that  involving  a  high  alternat- 
ing voltage  (Art.  206)  where- 
by an  insulation  breakdown 
may  be  produced,  should  be 
employed. 

210.  A  Rack  for  Testing 
Small  Armatures  using  the 
methods  of  Figs.  131,  132  and  134  is  detailed  in  Fig.  138.  Or- 
dinarily a  man  and  a  helper  are  necessary  in  testing  out  an 


Incandescent  U 


FIG.  138. — Rig  for  testing  small 
armatures.  (ELECTRICAL  REVIEW*) 


CutOut-. 


FIG.  139. — Showing  application  of  the  "modified  fuse  connector"  for 
connecting  test  lamp  in  series  in  the  testing  circuit. 

armature  but  with  the  appliance  shown  one    operator    can 
locate  the  faults. 

211.  A  Convenient  Arrangement  of  a  Test  Lamp  and  Ex- 
ploring Terminal  is  shown  in 
Fig.  139.  The  flexible  cord, 
and  the  indicating  lamp,  which 
also  serves  as  a  resistor  for 
limiting  the  current,  and  the 
exploring  terminal  can  be  cut 


Flexible  Co/r/-., 

Hard-Rubber 
Bushing— 


fibre  Tube-'' 


Terminal  8/ac/e- 


in  series  with  the  supply  cir-  FlG-  140.— Sectional  elevation  of 

fuse  connector. 

cuit  by  means  of  a  fuse  con- 
nector as  detailed  in  Fig.  140.     With  this  arrangement  the 
testing  set  can  be  quickly  cut  in  service  at  any  point  where 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     137 


there  is  a  cut-out.  Where  it  is  used,  it  may  be  necessary  to 
close  the  line  with  a  temporary  switch  or  otherwise  on  the 
load  side  of  the  cut-out.  An  Edison  plug  or  a  ferrule  contact 
fuse  can  be  used  instead  of  the  knife-blade  contact  fuse  of  Fig. 
140  by  adopting  obvious  modifications. 

212.  A  Poor  Connection  between  a  Bar  and  Coil  Leads 
will  cause  a  considerable  deflection  of  the  voltmeter  (Fig.  131) 
when  one  of  the  points  rests  on  the  bar  in  trouble  and  the 
other  rests  on  either  of  the  adjacent  bars. 

213.  An  Open-circuited  Coil,  as  h,  Fig.  131,  will  prevent 
the  flow  of  current  through  its  half  of  the  armature.     There 
will  be  no  deflection  on  that  half  of  the  armature  until  the 
"open"  is  bridged,  when  the  voltage  of  the  testing  circuit  will 
be  indicated. 

214.  Tests  for  Open  Armature  Circuits. — Another  method 
(Fig.  141)  is  to  apply  to  the  commutator,  at  two  opposite 
points,  a  low  voltage,  say  from  a  battery  or  from  a  dynamo 


Commutator 


-+To  Sourcg  of 

Low  Voltage 


*  Source  of 
Low  Volrage 

FIG.    141. — Testing    for  armature 
open-circuit  with  an  ammeter. 


Commutator-' 


Voltmeter 


FIG.  142. — Testing  for  armature 
open-circuit  with  a  voltmeter. 


with  a  suitable  resistance  (incandescent  lamps  for  example) 
in  series.  Place  an  ammeter  in  circuit  and  clean  the  surface 
of  the  commutator  so  that  it  is  bright  and  smooth.  The 
terminal  ends  leading  the  current  into  and  out  of  the  commuta- 
tor should  be  small,  so  that  each  rests  only  on  a  single  segment 
(Fig.  141).  Note  the  ammeter  reading  and  rotate  the  arma- 
ture slowly.  At  the  point  where  the  open  circuit  exists  the 
ammeter  needle  will  go  to  zero  if  the  leads  to  the  commutator 
bar  have  become  entirely  open-circuited.  This  is  because  the 
segment  is  attached  to  the  winding  through  the  commutator 
leads. 


138  ELECTRICAL  MACHINERY  [ART.  215 

If  the  armature  does  not  show  the  above  symptoms,  try  con- 
necting a  low-reading  voltmeter  or  a  galvanometer  to  two 
adjacent  segments  while  the  current  is  passing  through  the 
armature  as  described  from  some  external  low-voltage  source 
(Fig.  142).  Note  the  deflection.  Pass  from  segment  to  seg- 
ment in  this  manner,  recording  the  drop  between  the  succes- 
sive pair  of  bars.  This  drop,  if  the  current  is  maintained 
constant  from  the  external  source,  should  be  the  same  between 
each  pair  of  adjacent  segments.  If  any  pair  shows  a  higher 
drop  than  the  others  near  it,  a  higher  resistance  connection 
exists  there,  perhaps  causing  sparking  and  biting  of  the  com- 
mutator insulation,  to  a  less  degree,  to  be  sure,  than  with  an 
actual  open  circuit,  but  enough,  perhaps,  to  cause  the  trouble 
requiring  the  investigation. 

215.  The  Test  for  Armature  Short-circuits,  described  in  the 
preceding  paragraph  (Art.  204)  is  called  a  "bar  to  bar"  test. 
It  is  most  valuable  in  locating  faults  in  armatures.     It  is  the 
method  to  use  if  a  short-circuit  from  one  segment  to  another 
is  suspected.     When  the  section  in  which  the  short-circuit,  or 
partial  short-circuit,  exists  comes  under  the  contacts,  a  low 
or  perhaps  no  deflection  is  shown  on  the  galvanometer  or  volt- 
meter, thus  locating  the  defective  place.     Such  short-circuits, 
if  they  occur  when  running,  owing  to  defective  insulation, 
burn   out   the    coil   short-circuited.     When   the    coil  passes 
through  the  active  field  in  front  of  the  pole-piece,  an  immense 
current  is  induced  in  it,  causing  a  destruction  of  the  insulation. 
When  this  occurs  the  coil  should  be  open-circuited  if  the  burn- 
ing has  not  already  short-circuited  it.     If  practical,  it  should 
be  bridged  over,  as  suggested  in  a  preceding  paragraph. 

216.  If  Two  Bars  or  a  Coil  is  Short-circuited  as  at  /  and 
g  (Fig.  131)  respectively,  there  will  be  little  or  no  voltmeter 
deflection  when  the  two  bars  connecting  to  the  "  short-circuit " 
are  bridged  by  the  point. 

217.  A  Grounded  Armature  Coil  can  be  detected  in  practi- 
cally the  same  manner  as  indicated  in  Fig.  143  for  a  field  coil. 
Fig.  144  shows  the  connections.     Impress  full  voltage  on  the 
terminals  A  and  B,  clamped  to  the  commutator.     Ground  one 
side  of  the  voltmeter  on  the  shaft  or  spider  as  at  G  and  touch 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     139 


a  contactor  C  connected  to  the  other  side  of  the  voltmeter  to 
all  the  bars  in  succession.  The  minimum  deflection  will  ob- 
tain when  the  bars  connecting  to  the  grounded  coil  are  touched. 
A  battery  of  dry  cells,  E  (Fig.  144),  having  in  series  a  resistor, 
R,  may  be  used  as  a  source  of  energy.  A  bank  of  lamps 
connected  in  parallel  will  serve  as  a  resistor.  Instead  of  using 
a  voltmeter  as  in  Fig.  142  a  galvanoscope,  F  (Fig.  144) ,  may 
be  used.  Such  a  galvanoscope  may  be  improvised  by  wind- 
ing a  coil  of  wire  around  a  pocket  compass  but  where  this 
device  is  employed  the  galvanoscope  must  be  used  far  enough 
away  from  the  machine  under  test  so  that  the  compass  needle 
will  not  be  affected  by  the  magnetization  of  the  machine  under 
test. 


\ — 


or  Galvanometer 


FIG.  143. — Locating  a  grounded 
field  coil. 


FIG.  144. — Arrangement  of  con- 
nections for  locating  a  ground  in 
an  armature. 


218.  Crossed  Coil  Leads  as  at  a  (Fig.  131)  are  indicated  by 
a  twice  normal  deflection  when  the  points  bridge  the  bars  to 
which  the  crossed  coils  should  rightly  connect.     The  crossing 
of  the  coil  leads  connects  two  coils  in  series,  hence  causes  twice 
normal  drop.     Bridging  the  bars  to  which  coil  h  connects  will 
produce  a  normal  deflection,  but  it  will  be  reversed  in  direction. 

219.  Reversed  Armature   Coil. — Instead  of  the  armature 
winding  progressing  uniformly  around  from  bar  to  bar  of  the 
commutator,  there  may  at  some  point  be  a  coil  connected  in 
backward.     Such  a  reversed  coil  of  ten  -  causes  bad  sparking. 
One  way  to  locate  such  a  trouble  is  to  pass  a  current  through 
the  armature,  at  opposite  points  on  the  commutator.     Then 


140  ELECTRICAL  MACHINERY  [ART.  220 

with  a  compass  explore  around  the  armature  the  direction  of 
magnetism  from  slot  to  slot.  If  a  coil  is  reversed  when  the 
compass  comes  before  it,  the  needle  will  reverse,  giving  a  very 
definite  indication  of  the  improperly  connected  coil. 

220.  Heating  of  Armature.  * — Heating  of  the  armature  may 
develop  from  any  of  the  following  causes:   (a)  Too  great  a 
load;  (6)  a  partial  short-circuit  of  two  coils  heating  the  two 
particular  coils  affected;  (c)  short-circuits  or  grounds  on  arma- 
ture or  commutator. 

221.  Hot  Armature  Coils. — Sometimes  when  a  new  machine 
is  started,  local  heating  occurs  in  the  armature,  following  the 
exact  shape  of  the  armature  coil.     This  may  be  because,  in 
receiving  its  final  turning  off,  the  commutator  bars  were 
bridged  with  copper  from  one  segment  to  another  by  the 
action  of  the  turning  tool.     An  examination  of  the  commutator 
surface  will  reveal  this  bridging.     When  it  is  removed,  satis- 
factory operation  will  ensue  if  the  trouble  has  not  gone  too  far 
and  seriously  injured  the  insulation  of  the  coil. 

222.  "Flying  Grounds,"  "Flying  Short-circuits"  and  "Flying 
Open  Circuits"  sometimes  occur  in  armature  windings.    These 
are  intermittent  troubles  which  may  assert  themselves  when 
the  machine  is  hot,  after  running  underload,  and  its  component 
parts  expanded.     Then,  when  it  cools,  the  trouble  may  auto- 
matically correct  itself.     Sometimes  the  reverse  condition  oc- 
curs, that  is,  the  fault  will  be  present  when  the  machine  is  cold, 
but  will  be  temporarily  corrected  when  it  is  hot.     Further- 
more, centrifugal  force  due  to  the  rotation  of  the  armature 
may  be  a  factor  in  the  situation,  in  which  case  the  defect 
may  be   present  when    the  armature  is  rotated  at  normal 
speed,  but    not   present   when   the   armature  is  stationary. 
Obviously,     such     troubles    are    very     difficult     to     locate. 
About  the  only  way  to  localize  them  is  to  run  the  machine 
until  the  defect  has  "burnt  itself   out"   or  otherwise   ren- 
dered its  presence  sufficiently  evident  that  it  can  be  located 
by  the  usual  testing  methods  described  above. 

223.  A  Grounded   Field   Coil  can  be  Located  (Fig.  143) 
by  connecting  a  source  of  voltage  to  the  machine  terminals 

*  WESTINQHOUSE  INSTRUCTION  BOOK. 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     141 

having  first  raised  the  brushes  from  the  commutator,  if  it  is 
a  direct-current  machine.  Connect  one  terminal  of  the  volt- 
meter to  the  frame  and  the  other  to  a  lead  with  a  bared  end. 
Tap  with  the  bared  end  exposed  parts  of  the  field  circuit.  The 


Connection  to 
Frame 


•Part  of  Machine  Frame 


Voltmeter 


FIG.  145. — Connections  for  the  determination  of  the  insulation  resistance 
of  one  field  coil. 

voltmeter  deflection  will  be  least  near  the  grounded  coil. 
The  insulation  resistance  of  a  field  coil  may  be  determined 
with  the  arrangement  of  Fig.  145. 

224.  A  Method  of  Locating  an  Open-circuited  Field  Coil  is 
illustrated  in  Fig.  146.  Con- 
nect one  terminal  of  the  volt- 
meter to  one  side  of  the  field- 
coil  circuit  and  with  the  bared 
end  of  a  wire  or  a  contactor, 
successively  touch  the  junc- 
tions of  the  field-coil  leads 
around  the  frame.  When  the 
open  coil  is  bridged  the  volt- 
meter will  show  a  full  deflec-  __ 

tion.     Another  way:   Connect  _ 

, ,       _  .  ,       ..  . .  .      ,     FIG.  146. — Locating  an  open  field 

the  field-coil  circuit  terminals  coil. 

to  a  source  of  voltage.     Con- 
nect the  voltmeter  successively  across  each  coil  as  indicated 
by  the  dotted  lines  in  Fig.  146.     There  will  be  no  deflection 


u-'" Field  Coil  Circuit 
Terminals 


Field 
Coil 


142  ELECTRICAL  MACHINERY  [ART.  225 

on  the  voltmeter  until  the  open  coil  is  bridged,  when  the  full 
voltage  of  the  circuit  will  be  indicated. 

225.  Open  Field  Circuit. — If,  on  closing  the  field  switch,  no 
magnetization  is  detected  by  trial  with  any  piece  of  iron,  a 
key  for  example,  there  is  an  open  circuit  within  one  of  the 
spools  or  in  the  wires  leading  to  these  spools.     The  open  cir- 
cuit can  be  located  by  cutting  out  one  spool  at  a  time  and 
allowing  current  to  flow  through  the  rest  until  the  defective 
spool  is  discovered.     On  a  two-pole  motor  try  first  one  spool 
and  then  the  other.     For  a  very  short  time,  say,  10  min., 
double  voltage  can  be  carried  on  a  spool.     On  a  motor  having 
four  or  more  poles,  three  spools  can  always  be  left  in  circuit 
during  the  search  for  the  open-circuit. 

226.  Heating  of  Field  Coils*  may  develop  from  any  of  the 
following  causes:  (a)  Too  low  speed;  (b)  too  high  voltage;  (c) 
too  great  forward  or  backward  lead  of  brushes;  (d)  partial 
short-circuit  of  one  coil ;  (e)  overload. 

227.  Reversed  Field-spool  Connection. — There  may  be  cases 
where  the  manufacturer  has  shipped  a  motor  with  one  or  more 
field  spools  reversed.     If  such  is  the  case  no  torque,  or,  per- 
haps, very  weak  torque,  will  result.     Under  such  conditions 
a  trial  with  a  key  or  other  piece  of  iron  will  show  proper 
field  magnetism,  yet  the  weakness  or  total  absence  of  torque 
will  be  present,  and  a  trial  for  polarity  (Art.  98)  should  be 
made. 

228.  If  a  Direct-current  Motor  will  not  Start  When  the 
Starting  Box  is  Operated  and  when  current  is  flowing  in  the 
armature,  an  investigation  should  be  made  to  see  if  there  is  a 
field  flux.     This  can  be  done  by  holding  a  piece  of  iron,  such 
as  a  key,  against  the  pole-piece.     If  the  flux  exists  the  key  will 
be  drawn  strongly  against  the  pole-piece;  if  there  is  no  flux 
there  will  be  practically  no  attraction. 

229.  Sometimes  a  Motor  When  Started  will  Run  in  the 
Wrong  Direction. — The  only  change  necessary  is  to  reverse  the 
field  connection.     Thus  Fig.  147  shows  at  the  right  the  con- 
nection for  one  direction  of  rotation  and  at  the  left,  that  for  the 
other.     Note  that  in  Fig.  147  7,  the  brushes  A  and  A1  are 

*  WESTINOHOUSE  INSTRUCTION  BOOK. 


4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     143 


shifted  backward  against  the  direction  of  rotation.  For  the 
opposite  rotation,  a  backward,  lead  as  shown  in  Fig.  147,  //, 
must  be  chosen. 


Armature  ' 

Right  Hand   Rotation  Left     Hand  ^Rotation. 

FIG.  147. — Connections  for  shunt-wound  motors. 

230.  Bearing    Troubles    of    Motors    and    Generators. — 

Modem  generators  and  motors,  both  alternating-  and  direct- 
current,  have  self-oiling  or 
(for  some  machines  of  the 
smaller  capacities)  ball  bear- 
ings, (Figs.  148  and  149). 
The  self-oiling  bearings  should 
be  filled  to  such  a  height  that 
the  rings  will  carry  sufficient 
oil  upon  the  shaft.  If  the 
bearings  are  too  full,  oil  will 
be  thrown  out  along  the  shaft. 
Watch  the  bearings  carefully 
from  the  time  the  machine  is 
first  started  until  the  bearings 
are  warmed  up,  then  note  the 
oil  level.  The  expansion  of 


the  oil  due  to  heat  and  foam- 
ing raises  the  level  consider- 
ably during  that  time.  The 
oil  should  be  renewed  about 

once  in  six  months,  or  oftener  if  it  becomes  dirty  or  causes 

the  bearings  to  heat. 


FIG.  148. — Showing  the  con- 
struction of  self -oiling  bearings 
used  in  small-capacity  Lincoln 
Electric  Company  motors. 


144 


ELECTRICAL  MACHINERY 


[ART.  231 


231.  Bearings  Must  be  Kept  Clean  and  Free  from  Dirt.— 

They  should  be  examined  frequently  to  see  that  the  oil  supply 
is  properly  maintained  and  that  the  oil  rings  do  not  stick. 
Use  only  the  best  quality  of  oil.  New  oil  should  be  run  through 
a  strainer  if  it  appears  to  contain  any  foreign  substances.  If 
the  oil  is  used  a  second  time  it  should  first  be  filtered  and,  if 
warm,  allowed  to  cool.  If  a  bearing  becomes  hot,  first  feed 
heavy  lubricant  copiously,  loosen  the  nuts  on  the  bearing  cap, 
and  then,  if  the  machine  is  belt-connected,  slacken  the  belt. 
If  no  relief  is  afforded  by  these  means,  shut  down,  keeping 
the  machine  running  slowly  until  the  shaft  is  cool,  in  order 


-Lifting  Loop 


.•Oil  Jhroner$ 


on 

Rings. 


-Shaft 


-^  Oil  Level—  — 


Spherfca/- 
Seat 


FIG.  149. — Showing  the  construction  of  a  modern,  ring-oiling  bearing  for 
a  medium,  and  large-capacity  electrical  machine. 

that  the  bearing  may  not  " freeze."  Renew  the  oil  supply 
before  starting  again.  A  new  machine  should  always  be  run 
at  a  slow  speed  for  an  hour  or  so  in  order  to  see  that  it  oper- 
ates properly.  The  bearings  should  be  inspected  at  regular 
intervals  to  insure  that  they  always  remain  in  good  condition. 
The  higher  the  speed,  the  more  care  should  be  taken  in  this 
regard. 

232.  A  Warm  Bearing  or  "Hot  Box"  is  probably  due  to 
one  of  the  following  causes:     (1)  Excessive  belt  tension.     (2) 
Failure  of  the  oil  rings  to  revolve  with  the  shaft.     (3)  Rough 
bearing  surface.  •  (4)  Improper  lining  up  of  bearings  or  fitting 
of  the  journal  boxes. 

233.  Ball  Bearings  are  now  being  used  for  electrical  ma- 
chinery, particularly  for  units  of  medium  and  small  capacity. 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     145 

Typical  examples  of  these  applications  are  shown  in  Figs.  150, 
151,  and  152.     Ball  bearings  involve  less  friction  loss  than  do 


•Cap     . 
-BallBearing 


FIG.  150. — Gurney  ball  bearings  in  a  vertical  induction  motor.     With 
this  arrangement  no  thrust  bearings  are  necessary. 


'Stator  Winding 
^'Ba//  Bearing 


'-End  Housing 


FIG.  151. — Application  of  ball  bearings  to  a  polyphase  squirrel-cage 
induction  motor.     (Gurney  Bearing  Co.) 

those  of  the  babbitted  type,  the  friction  loss  with  them  being 
approximately  one-quarter  of  that  which  obtains  with  bab- 


10 


146  ELECTRICAL  MACHINERY  [ART.  234 

bitted  bearings  of  good  construction.  This  means  that  where 
an  electrical  machine  is  fitted  with  ball  bearings  its  bearing 
friction  loss  will  be  negligible.  Another  desirable  feature  of 
ball  bearings  is  that  because  of  their  very  small  friction  the 
wear  in  them  is  very  slight  so  that  low  bearing  troubles  and 
variations  in  air  gap  due  to  wear,  which  is  encountered  with 
babbitted  bearing  machines  after  they  have  been  in  service 
for  a  considerable  period,  is  greatly  minimized.  Experience 
has  indicated  that  ball  bearings,  if  properly  designed  and  prop- 


Bctll- 

Bearing 


FIG.  152. — Showing  application  of    Gurney  ball-bearings  in  a  direct- 
current,  train-lighting  generator. 

erly  applied,  will  show  no  measurable  wear  after  many  years 
of  continued  service.  Where  bearings  of  this  type  are  used 
on  vertical  motors  (Fig.  150),  if  suitably  designed,  individual 
thrust  bearings  are  unnecessary,  which  tends  toward  a  re- 
duction in  weight  and  initial  cost.  Good  ball  bearings  need 
no  other  attention  than  a  change  of  the  lubricating  oil  a  couple 
of  times  a  year. 

234.  To  Insure  That  an  Armature  is  Properly  Balanced — 
if  it  is  not  balanced  it  will  probably  cause  noise  when  rotating 
at  normal  speed — it  is  necessary  to  remove  the  armature  from 
the  machine.  It  is  then  placed  on  a  balancing  stand  or  on 


SEC.  4]  DIRECT-CURRENT  GENERATORS  AND  MOTORS     147 


level  knife-edge  bars  (Fig.  153)  to  ascertain  which  is  the  heavy 
side.  If  the  armature  is  properly  balanced,  it  will  each  time, 
after  having  been  rotated  by  hand,  stop  in  no  particular  posi- 
tion, but  if  it  is  out  of  balance  it  will  always  stop  in  the  same 


Armafure 
Shaft 


Ntfe:  Knife  Edges  should 
be  perfectly  level 
and  parallel 


Shaft, 


•Armature 


I-Roller-Bearing  Stand  I- Knife  Edges  for  Balancing 

FIG.  153. — Roller-bearing  armature  stand  and  knife-edge  leveling  rig. 

position  on  the  balancing  rig  (Fig.  153),  with  the  heavy  side 
down.  The  counterbalancing  weight  which  should  be  added 
to  the  light  side  to  properly  balance  the  member  may  consist 
of  a  screw  which  is  turned  into  a  tapped  hole  in  some  iron 
part  of  the  commutator  struc- 
ture. By  filing  just  the  proper 
amount  of  metal  from  the  screw 
head,  very  accurate  balance  will 
be  obtained.  Or  instead,  a 
small  ball  of  solder  may  be 
sweated  to  the  light  side  or 
some  of  the  metal  filed  from 
the  heavy  side.  Ascertain  by 
trial  just  how  much  metal  should  be  added  or  removed  be- 
fore attempting  to  make  a  permanent  correction. 

235.  A  Rack  for  Supporting  Medium-size  Armatures  may 
be  constructed  as  shown  in  Fig.  154.  Such  an  outfit  will  not 
alone  be  found  very  convenient  but  will  likely  prove  a  profit- 
able investment  in  that  if  used  it  will  tend  to  prevent  damage 
to  the  armature  winding  and  end  connections. 


'Wing  Nut 
•fruiafe  Irons 

FIG.  154. — A  rack  for  medium-size 
armatures.     (ELEC.  REV.) 


SECTION  5 

TESTING  OF   DIRECT-CURRENT  GENERATORS  AND 

MOTORS 

236.  Motor  Testing. — In  all  of  the  discussion  of  this  sub- 
ject, of  testing  for  both  direct-current  and  alternating-cur- 
rent motors,  it  is  assumed  that  the  motor  is  loaded  in  the 
usual  way  by  belting  or  direct-connecting  it  to  some  form 
of  load,  and  that  the  object  is  then  to  determine  whether  the 
motor  is  over-  or  underloaded,  and  approximately  what  per 
cent,  of  full-load  it  is  carrying.     All  commercial  motors  have 
nameplates,  giving  the  rating  of  the  motor  and  the  full-load 
current  in  amperes.     The  per  cent,  of  full-load  carried  by  the 
machine   can,   therefore,   be    determined    approximately  by 
measuring  the  current  input  and  the  voltage.     If  an  efficiency 
test  of  the  apparatus  is  required,  it  becomes  necessary  to  use 
some  form  of  absorption  dynamometer,  such  as  a  Prony  (Art. 
239)  or  other  form  of  brake.     The  output  of  the  motor  can 
then  be  determined  from  the  brake  readings.    The  accuracy 
of  all  tests  is,  obviously,  dependent  upon  the  accuracy  of  the 
instruments  employed.     Before  accepting  the  results  obtained 
by  any  test,  particularly  under  light  or  no-load,  one  should 
be  certain  that  the  instruments  employed  are  accurate  under 
the  conditions  encountered. 

237.  The  Horse-power,  Torque  and  Speed  Formulas  For 
All  Electric  Motors  follow  from  the  general  formula  for  horse- 
power, the  derivation  of  which  is  suggested  in  Art.  238.     Thus, 

2  X  3.14  X  T  X  r.p.m.      6.28  X  T  X  r.p.m. 
33,000  33,000  " 

Then,  dividing  both  numerator  and  denominator  of  (17)  as 
simplified,  by  6.28: 

(18)  h.p.  =  — 5  £52™  (horse-power) 

148 


SEC.  5]        TESTING  OF  GENERATORS  AND  MOTORS 


149 


and 
(19) 
hence 

(20) 


r.p.m.  = 


5,252  X  h.p. 
r.p.m. 

_  5,252  X  h.p. 


(pound-foot) 
(rev.  per  min.) 


Wherein,  h.p.  =  horse-power  output  of  the  motor,  when  it  is 
running  at  the  r.p.m.  speed  and  developing  torque  T.  T  = 
torque  in  pounds-feet,  or,  say- 
ing the  same  thing  in  another 
way,  the  torque  in  pounds  at 
1-ft.  radius,  r.p.m.  =  speed  of 
motor,  in  revolutions  per 
minute. 

238.  How     To     Determine 
the  Horse -power  Output  of  an 
Electric  Motor  with  a  Prony 
Brake  is  explained  in  the  fol- 
lowing    example     (Fig.    155). 
The  principle  involved  has  as 

its  basis  the  concept  of  torque  FIG.  155.— Arrangement  for  de- 
which  is  discussed  in  the  au-  J™§htl>r^  bSf  *  * 
thor's  PRACTICAL  ELECTRICITY. 

EXAMPLE. — The  torque  (Fig.  155)  is  20  Ib.  at  4-ft.  radius,  or  80  lb.-ft.T 
or  80  Ib.  at.  1-ft.  radius.  Since  the  motor  pulley  is  turning  at  the  rate 
of  1,000  r.p.m.,  a  point  on  its  circumference  travels:  2  X  if  X  radius  X 
r.p.m.  =  2  X  3.14  X  1  X  1,000  =  6,280  ft.  per  min.  At  its  circumfer- 
ence the  pulley  is  then  "overcoming"  a  resistance  of  80  Ib.  Therefore, 
it  is  doing  work  at  the  rate  of :  80  X  6,280  =  502,400  ft.-lb.  per  min. 
Since,  when  work  is  done  at  the  rate  of  33,000  ft.-lb.  per  min.,  a  horse- 
power is  developed,  the  motor  is  then  delivering:  502,400  •£•  33,000  = 
15.2  h.p.  It  should  be  noted  that,  though  the  torque  at  the  circumfer- 
ence of  the  motor  pulley  was  considered  in  this  example,  it  is  not  neces- 
sary to  take  the  torque  at  this  point.  The  torque  may  be  taken  at  any 
point,  if  the  radius  to  that  point  is  used,  instead  of  the  radius  of  the 
pulley.  Substituting  the  values  from  the  above  example  in  formula  (18) 
of  Art.  237:  h.p.  =  (80  X  1,000)  -r-  5,252  =  15.2  h.p.  This  is  the  same 
result  secured  by  the  former  and  longer  method. 

239.  There  are  Several  Different  Forms  of  Prony  Brakes, 
the  most  important  of  which  are  illustrated  in  Figs.  155,  156 


150 


ELECTRICAL  MACHINERY 


[ART.  239 


and  157.  With  the  brakes  of  Figs.  155  and  156:  torque,  in 
pounds-feet  =  Fp  X  L.  Where  Fp  =  force,  measured  in  pounds, 
at  the  end  of  the  brake  arm;  and  L  =  the  effective  length 


///////7/////////^^^ 
FIG.  156. — Prony  brake  of  the  band-brake  type. 

of  the  brake  arm,  in  feet,  that  is,  the  distance  from  the  center 
of  the  shaft  to  the  point  at  which  the  force  is  measured.  For 
measuring  small  torques,  the  self-regulating  brake  (Fig.  157,  /) 


I-  180  Degree  Wrap 


-  36°  Degree  Wrap 


FIG.  157.  —  Different  forms  of  prony  brakes. 

may  be  arranged  by  setting  copper  rivets  in  a  leather  belt. 
The  rivets  per  unit  of  surface  should  increase  from  zero  at 
one  end  of  the  working  surface  to  a  maximum  at  the  other. 


SEC.  5]        TESTING  OF  GENERATORS  AND  MOTORS 


151 


Where  very  small  torques  are  to  be  measured,  a  round  belt 
in  a  grooved  pulley  can  be  used  for  the  band.*  The  belt 
should  have  a  small  steel  wire  wound  around  it  spirally.  The 
turns  of  the  wire  around  the  belt  should  increase  from  zero 
at  one  end  to  a  maximum  number  at  the  other. 

With  the  arrangement  of  Fig.  157,  7 :  T  =  (Fwl  -  F pi)  X  Iu. 
The  distance,  LI,  should  be  measured  from  the  center  of  the 
shaft  to  the  center  of  the  band  or  belt.  At  //:  T  =  (Fw2  -  FpZ) 
X  Lz.  With  the  brake  of  III:  T  =  Fp3  X  L3.  In  all  of  these 
determinations,  if  F  and  L  are  measured  in  pounds  and  feet, 
respectively,  T  will  be  the  torque  in  pounds-feet. 

240.  A  Direct-current  Motor  or  Generator  Magnetization- 
curve  Test  may  be  conducted  as  shown  in  Fig.  158.  See  Fig. 
18  for  a  magnetization  curve.  The  object  of  this  test  is  to 


FIG.  158. — Arrangement  for  obtaining  data  for  plotting  a  magnetization 
graph  for  a  direct-current  motor  or  generator. 

determine  the  variation  of  armature  voltage,  without  load  on  the 
machine,  with  different  intensities  of  current  flowing  through 
the  field  circuit.  The  armature  should  be  driven  at  normal 
speed.  The  effective  resistance  of  the  rheostat,  R,  in  the  field 
circuit  is  varied  and  the  voltage  across  the  armature  measured. 
The  curve  obtained  by  plotting  a  series  of  these  two  values 
is  usually  called  magnetization  curve  or  graph  of  the  generator. 
It  is  usual  to  start  with  a  high  resistance  in  the  field  circuit 
so  that  very  small  field  current  flows,  gradually  increasing  this 
current  by  cutting  out  the  field  resistance.  When  the  highest 
no-load  voltage  required  is  attained,  the  field  current  is  then 
diminished,  and  the  data  for  what  is  called  the  descending  (as 
opposed  to  the  ascending)  magnetization  curve  are  obtained. 
The  difference  in  the  two  curves  is  due  to  the  lag  of  the 

•  H.  N.  Scheibe,  ELECTRIC  JOURNAL;  vol.  iv,  p.  118. 


152 


ELECTRICAL  MACHINERY 


[ART.  241 


magnetization  behind  the  magnetizing  current,  and  is  due  to 
the  hysteresis  of  the  iron  in  the  armature  core. 

241.  A  Load  and  Speed  Test  of  a  Direct-current  Shunt 
Motor  may  be  made  with  the  equipment  diagrammed  in  Fig. 
159.  The  procedure  in  this  test  is  to  maintain  the  voltage 
applied  to  the  motor  constant,  and  to  vary  the  load  by  means 
of  a  Prony  brake  (Art.  238)  and  determine  the  corresponding 
variation  in  speed  of  the  machine  and  in  the  current  drawn 
from  the  supply  circuit.  If  the  motor  is  a  constant-speed 
machine  the  field  resistance  is  maintained  constant.  For  start- 
ing the  machine,  an  ordinary  starting  rheostat  should  be 
inserted. 


FIG.  159. — Connections  for  a  load  and  speed  test  on  a  direct-current 

shunt  motor. 

242.  In  Making  a  Temperature  Test  of  a  Direct-current 
Shunt  Motor  or  Generator  by  the  "Loading-back"  Method 

the  equipment  may  be  disposed  as  illustrated  in  Fig.  160.  In 
making  temperature  tests  on  a  small  generator  (this  method 
is  not  illustrated)  it  is  usual  to  drive  the  generator  with  a 
motor  and  to  load  the  generator  with  a  lamp  bank  or  resistance, 
the  voltage  across  the  generator  being  maintained  constant, 
and  the  current  through  the  external  circuit  adjusted  to  full- 
load  value.  The  temperatures  are  then  recorded.  When  they 
reach  a  constant  value  above  the  temperature  of  the  atmos- 
phere, the  test  is  discontinued.  Similarly  in  making  a  test 
on  a  small  motor,  the  motor  is  loaded  with  a  generator  and 
the  load  increased  until  the  input  current  reaches  the  normal 
full-load  value  of  the  motor,  the  test  being  conducted  as  for 
a  small  generator. 

When,  however,  the  apparatus,  either  motor  or  generator, 
is  of  considerable  capacity  it  becomes  necessary,  in  order  to 


SEC.  5]        TESTING  OF  GENERATORS  AND  MOTORS 


153 


economize  energy  and  thus  decrease  the  cost  of  testing,  to  use 
what  is  called  the  loading  back  method.  The  apparatus  for 
this  is  shown  in  Fig.  160.  The  motor  is  started  in  the  usual 
way,  with  the  generator  belted  to  it,  the  external  circuit  of  the 
generator  being  open.  The  field  current  of  the  generator  is 
then  so  adjusted  that  the  generator  voltage  is  equal  to  that  of 
the  line.  The  generator  is  then  connected  to  the  circuit  and 
its  field  resistance  varied  until  it  carries  normal  full-load  cur- 
rent, or  slightly  less  than  full-load  current.  Under  these  con- 
ditions, if  the  motor  and  generator  are  of  the  same  type  and 
size,  the  motor  will  carry  slightly  in  excess  of  full-load,  the 


Motor  Generator 

FIG.  160. — Connections  for  temperature  test  of  shunt  generator  or  motor 
by  the  "load-back"  method. 

difference  being  approximately  twice  the  losses  of  the  ma- 
chines. Under  these  conditions  the  total  power  drawn  from 
the  line  is  equal  to  twice  the  loss  of  either  machine.  Tem- 
perature readings  are  taken  as  in  other  temperature  tests. 

243.  The  Determination  of  the  External  Characteristic  of  a 
Direct-current,  Compound-wound  Generator  Under  Adjust- 
able Load  may  be  effected  with  apparatus  arranged  as  shown 
in  Fig.  161.  The  object  of  this  test  is  to  determine  the  relation 
between  armature  voltage  and  armature  current  when  the 
shunt-field  current  is  maintained  constant.  The  shunt  field 
is  adjusted  to  give  normal  voltage  across  the  armature  when 
the  external  circuit  is  open.  The  load  is  then  imposed  by 


154 


ELECTRICAL  MACHINERY 


[ART.  244 


means  of  an  adjustable  resistance  or  lamp  bank,  R,  and  read- 
ings of  external  voltage  and  current  recorded.  If  the  machine 
is  normally  compounded  (Fig.  15)  the  external  voltage  will 
remain  practically  constant  throughout  the  load  range.  If 
the  machine  is  under-compounded,  the  external  voltage  will 
drop  with  load,  while  if  over-compounded,  (Fig.  14)  there 
will  be  a  rise  in  voltage  with  increase  in  load. 


.'Armature 


Ammeter'' 

> 'Adjustable 
Load  (Water 

Rheostat  or        Voltmeter-'7 
Other  Variable 
Resistance) 


m- 

&£ 

'Field  Ammeter 
field  Rheostat 

_/X/wwv 


FIG.  161. — Testing   arrangement   for   determining   the   external   char- 
acteristic of  a  compound-wound  direct-current  generator. 

244.  A  Test  to  Determine  the  External  Characteristic  of  a 
Shunt-wound  Direct-current  Generator  is  diagrammed  in  Fig. 
162.  The  external  characteristic  of  a  shunt  generator  is  a 
graph  (Fig.  10,  /)  showing  the  relation  between  the  current  and 
voltage  of  the  external  circuit.  The  shunt-field  current  is  so 
adjusted  by  manipulating  rheostat,  R,  that  the  machine  im- 


FIG.  162. — Arrangement  for  obtaining  data  for  external  characteristic 
of  a  shunt-wound  generator. 

presses,  on  its  external  circuit,  normal  voltage  when  the  ex- 
ternal circuit  is  open.  The  field  current  is  then  maintained 
constant  and  the  external-circuit  current  varied  by  varying 
the  resistance  in  the  circuit  with  the  rheostat,  L.  By  plotting, 
on  squared  paper,  voltage  along  the  vertical,  against  the  corre- 
sponding amperes  along  the  horizontal,  the  external  character- 
istic graph  is  obtained. 


SEC.  5]        TESTING  OF  GENERATORS  AND  MOTORS  155 

245.  Measurement  of  the  Insulation  Resistance  of  Genera- 
tors and  Motors  will  give  an  indication  of  the  average  condi- 
tion of  the  insulation  as  regards  moisture  and  dirt,  but  will 
not  always  detect  weak  spots.*  The  higher  the  insulation 
resistance,  the  better  the  general  condition  of  the  insulating 
material.  The  approximate  figure  of  1  megohm  per  1,000  volts 
of  rated  e.m.f.  when  the  machine  is  at  its  normal  full-load 
temperature  may  be  taken  as  indicating  a  fairly  satisfactory 
condition  of  the  armature  insulation.  The  insulation  resist- 
ance of  the  field  will  be  much  higher  in  proportion  to  the  e.m.f. 
of  the  exciting  current  and  will  seldom  give  appreciable 
trouble.  Since  large  armatures  have  much  greater  areas  of 
insulation,  their  insulation  resistance  will  be  proportionally 
lower  than  that  of  small  machines.  Even  though  the  material 

Source  of  Voltage 

'I 

Frame 


t 

—  c  o— 
-o  o— 

T 

Double  fo/e, 
Double  Throw  Switch 

dh 

7o  Resistance  tobe  measured^ 

^—  K0//777e/Er  Winding^ 

FIG.  163. — Measuring  generator  insulation  resistance. 

is  in  exactly  the  same  condition,  the  insulation,  resistance  of 
any  machine  will  be  much  lower  when  hot  than  when  cool, 
especially  when  the  machine  is  rapidly  heated. 

246.  The  Only  Feasible  Way  of  Increasing  the  Insulation 
Resistance  When  the  Machine  is  Complete  is  by  "Drying 
Out." — Armature  winding  and  field  coils  are  dried  out  by  heat; 
baking  in  an  oven  is  to  be  preferred,  but  is  often  impracticable. 
They  are  usually  heated  by  the  passage  of  current.     In  the 
case  of  the  armature  this  may  be  accomplished  by  short-cir- 
cuiting the  leads  and  running  the  generator  with  a  low  field 
charge,  just  sufficient  to  produce  the  proper  current.     See 
Art.  107. 

247.  Insulation  Resistance  may  be  Conveniently  Measured 
with  a  High-resistance  Voltmeter,  preferably  one  especially 

*  Westinghouse  Elec.  &  Manfg.  Co. 


156  ELECTRICAL  MACHINERY  [ART.  247 

designed  for  the  purpose.  Voltmeters  having  a  resistance  of 
1  megohm  are  now  made  for  this  service  so  that,  if  one  of 
these  instruments  is  used,  the  calculation  is  somewhat  simpli- 
fied. A  double-pole  switch  arranged  as  indicated  in  Fig.  163 
is  convenient  for  changing  the  voltmeter  connections.  If  a 
grounded  circuit  is  used  for  making  this  measurement,  care 
must  be  taken  to  connect  the  grounded  side  of  the  line  to  the 
frame  of  the  machine  to  be  measured,  and  the  voltmeter  be- 
tween the  windings  and  the  other  side  of  the  circuit.  Fig. 
145  illustrates  the  method  of  determining  the  insulation  resist- 
ance of  a  field  coil.  See  the  author's  AMERICAN  ELECTRICIAN'S 
HANDBOOK  for  a  description  and  examples  of  the  method  and 
figuring  used  in  making  determinations  of  insulation  resistance 
with  a  voltmeter. 


SECTION  6 

PRINCIPLES,  CONSTRUCTION  AND  CHARACTERISTICS 
OF  ALTERNATING-CURRENT  GENERATORS 

248.  Modern  Commercial  Alternating-current  Generators 
or  Alternators  usually  are  arranged  as  suggested  diagrammat- 
ically  in  Fig.  164.     Electromagnets,  excited  by  a  small  direct- 
current  generator  or  exciter,  are  mounted  on  a  wheel-like 
structure  which  revolves  within  a  circular  stationary  frame  in 
the  inner  surface  of  which  are  armature  coils.     The  revolving 
part  is  the  revolving  field;  the  stationary  part  is  the  armature. 
The  direct  current  is  fed  to  the  field  coils  through  collector 
rings.     Armature  coils  are,  in  practice,  arranged  in  slots  in 
the  inner  circumference  of  the  armature  structure.     Alternat- 
ing e.m.fs.  are  induced  in  the  armature  by  the  lines  of  force 
from  the  revolving  field  magnets  cutting  the  armature  coils. 
The  alternating  voltage  can  be  varied,  within  limits,  by  ad- 
justing the  field  rheostats. 

249.  There  are  Several  Types  of  Alternators  or  alternating- 
current  generators.     They  are:  (1)  Revolving-armature  alter- 
nators (Fig.  165)  wherein  the  armature  revolves  and  the  field 
magnets  are  stationary;  revolving-armature  machines  are  now 
ordinarily  manufactured  only  for  capacities  of  less  than  about 
30  kva. ;  (2)  revolving-field  alternators,  wherein  the  field  mag- 
nets revolve  and  the  armature  is  stationary  (Fig.  166);  (3) 
inductor  alternators,  wherein  both  field  magnets  and  armature 
are  stationary  and  iron  cores  revolve  between  the  armature 
core  and  the  field-magnet  poles.     Modern  alternators  of  mod- 
erate and  large  capacity  are  practically  all  of  the  revolving- 
field  type  because  the  stationary  armature  offers  better  op- 
portunity for  insulation  and  a  high  voltage  is  not  necessary 
on  the  collector  rings.     Fig.  166  shows  a  modern  installation 
of  a  small  revolving-field  alternating-current  generator.     In 

157 


158 


ELECTRICAL  MACHINERY 


[ART.  249 


I 
I 

s 


SEC.  6]         ALTERNATING-CURRENT  GENERATORS 


159 


Shaft 
Extended 
For 
Pulley 


'Revolving  Armature 
Collector  Rings 

-Brush 


^"•Direct-Connected 
Exciter 


--Pressed- Stee!  foot 


FIG.  165. — A  small  revolving-armature  generator.  (This  Westing- 
house  belted  alternator  is  rated  at  20-kva.,  three-phase  or  14-kva.  single 
phase,  60-cycles,  1800  r.p.m.,  120,  240,  480  or  600  volts,  three-  or  single- 
phase. 


FIG.  166. — A  Ridgway  75-kva.,  277-r.p.m.,  three-phase,  60-cycle,  240- 
volt,  direct-connected,  engine-type  generator,  driven  by  a  12  X  14 
106-h.p.  high  speed  Ridgway  engine.  The  exciter  is  belt-driven  and  has 
an  output  of  5  kw. 


160 


ELECTRICAL  MACHINERY 


[ART.  250 


Fig.  167  are  illustrated  two  revolving  field  turbo-generator 
units. 


FIG.  167. — Two  500  kva.,  Ridgway,  mixed-pressure,  turbo-alternator 
units.    2200  volts,  three-phase,  60-cycles. 


Hand 


Frame  - ' 


FIG.  168. — Stator  of  a  Westinghouse  alternating-current  generator  for 
steam-turbine  drive. 

250.  The  Construction  of  Turbo-alternators — that  is  alter- 
nating-current generators  arranged  for  steam-turbine  drive 


SEC.  6]         ALTERNATING-CURRENT  GENERATORS 


161 


*  Ventilating  Chimney 


Cast  Irortr 
Frame 


FIG.   169. — Stator  (alternating-current  armature)  of  a  turbo  generator 
without  armature  windings  and  with  end  bells  removed. 


Steel 
Core 


Raotial  Slots  Machined 

in  Solid  Core  for-^ 

Armature  Coifs        *«*—"_,-"  * 


FIG.  170. — Rotor  (direct-current  field)  core  for  an  alternating-current 
turbo  generator.     This  shows  one  of  the  "radial-slot"  type. 


11 


162 


ELECTRICAL  MACHINERY 


[ART.  250 


(Figs.  168  to  171) — is  somewhat  different  from  that  utilized  for 
machines  which  are  to  be  driven  at  moderate  speeds.  How- 
ever, these  are  always  revolving-field  machines.  Steam  tur- 
bines, if  they  are  to  develop  their  maximum  economies,  must 
operate  at  high  speeds  and  it  follows  that  generators  which 
are  to  be  driven  directly  by  them  must  be  capable  of  operating 


Ventilating. 
Holes 


FIG.  171. — End-view  of  a  turbo-alternator  armature  with  the  winding 
and  braces  in  position. 

at  the  same  speeds;  1,200,  1,800  and  3,600  r.p.m.  are  speeds 
frequently  encountered  in  practice.  These  machines  ordi- 
narily require  forced  ventilation,  hence  are  usually  enclosed 
as  shown  in  Fig.  168.  Since  the  rotors  (fields)  revolve  so 
rapidly,  they  must  be  of  very  secure  construction.  Hence, 
for  small  and  medium-capacity  machines,  the  field  structure 
is  frequently  machined  from  a  steel  forging  as  shown  in  Fig. 


SEC.  6]         ALTERNATING-CURRENT  GENERATORS 


163 


170.     The  field  core  for  the  large  machines  is  assembled  from 
dishes  cut  from  steel  slabs  or  plates. 


Roller  TnntST--, 
Bearing 


•<R$nKivabJe  Caff 


^Cruio/e  Put  ley  for 
Quarter  Turn  fxciferBe/t 


-Shaft 


FIG.  172. — Sectional  elevation  of  an  Electrical  Machinery  Company 
vertical  waterwheel  generator.  (See  Fig.  173  for  a  photographic  repro- 
duction.) 


FIG.  173. — Showing  an  Electrical  Machinery  Company's  vertical 
waterwheel  generator  with  a  belted  exciter.  (See  Fig.  172  for  a  sec- 
tional elevation  of  a  generator  of  this  type.) 

251.  A  Sectional  View  of  a  Vertical  Water-wheel  Genera- 
tor, as  manufactured  by  The  Electrical  Machinery  Company, 


164 


ELECTRICAL  MACHINERY 


[ART.  252 


is  shown  in  Fig.  172.  The  stator  rests  on  a  circular  cast-iron 
base  which  carries  a  split,  babbitted,  self-oiling,  steady  bearing 
below  the  rotor.  The  vertical  shaft  is  arranged  to  couple 
direct  to  the  waterwheel  shaft.  A  thrust  bearing  of  the  roller 
type  which  carries  all  of  the  revolving  parts  is  at  the  top. 
This  machine  is  shown  installed  in  Fig.  173. 

252.  The  Electromotive  Force  in  an  Alternator  is  Generated 
as  Suggested  in  Fig.  174. — As  each  field  coil,  D  for  instance, 
sweeps  past  the  armature  coils  the  lines  of  force  from  the  field 
coil  cut  the  armature  coils.  As  coil  D  passes  from  A  to  C 
an  alternating  e.m.f.  represented  by  the  graph  ABC  will  be 
generated  in  the  armature.  It  should  be  understood  that  in 


Field  Spider-^          ( 
FIG.  174. — Armature  and  field  structure  developed. 


Direct/on  of 
~  Movement 


commercial  alternators  the  armature  coils  are  set  in  slots  and 
differently  arranged  than  in  Fig.  174,  which  only  illustrates  a 
principle.  For  a  simple  but  rather  complete  explanation  of 
how  and  why  alternating  e.m.fs.  are  generated  by  alternators, 
see  the  author's  PRACTICAL  ELECTRICITY. 

253.  The  Relation  Between  the  Speed,  Frequency  and 
Number  of  Poles  of  any  Alternating-current  Generator  is  ex- 
pressed by  the  following  formulas: 

v  X  r.v.m  ,~  ^ 

f  =  -     12Q  (frequency) 

_  120  X/ 

r.p.m. 


(21) 
(22) 


(number  of  poles) 


(23) 


r.p.m. 


(speed) 


SEC.  6]         ALTERNATING-CURRENT  GENERATORS  165 

Wherein,  /  =  frequency,  in  cycles  per  second,  r.p.m.  =  revo- 
lutions per  minute  of  rotor,  p  =  the  number  of  field  poles. 
A  table  will  be  found  in  the  author's  AMERICAN  ELECTRICIANS' 
HANDBOOK,  showing  the  synchronous  speeds  for  alternating- 
current  generators  for  all  of  the  commonly  used  number  of 
poles  from  2  to  100,  and  for  frequencies  of  25  to  133  cycles. 
See  the  author's  PRACTICAL  ELECTRICITY  for  a  much  more 
complete  discussion  of  the  subject  of  frequency. 

EXAMPLE. — What  is  the  frequency  developed  by  a  two-pole  alter- 
nating-current generator  which  is  rotated  at  3,600  r.p.m.?  SOLUTION. — 
Substitute  in  the  above  equation  (21) :/  =  (p  X  r.p.m.)  -r-  120  =  (2 
X  3,600)  -J-  120  =  7,200  -=-  120  =  60  cycles  per  sec. 

EXAMPLE. — How  many  poles  must  a  25-cycle  alternator  which  operates 
at  100  revolutions  per  minute  have?  SOLUTION. — Substitute  in  the 
equation  (22) :  p  =  (120  X  /)  -5-  r.p.m.  =  (120  X  25)  •*-  100  =  3,000 
-r-  100  =  30  poles. 


Armature' 

•Spinier  Arms  — • ? 

One  Slot  Rer  Pole  Two  Slots  per  Pole. 

FIG.  175. — Single-phase  armature  windings. 

254.  Single-phase  Alternators. — The  circumferential  dis- 
tance from  the  center  line  of  one  pole  to  the  center  line  of 
the  next  pole  of  the  same  polarity  constitutes  360  magnetic 
degrees.  See  Fig.  174,  which  shows  how  a  single-phase  e.m.f. 
is  generated.  See  the  author's  PRACTICAL  ELECTRICITY  for 
explanations  of  the  terms  " single-phase,"  "two-phase"  and 
" three-phase."  Fig.  164  is  a  diagrammatic  illustration  of  a 
single-phase  alternator  and  Fig.  175  shows,  diagrammatically, 
two  different  kinds  of  single-phase  windings.  Single-phase 
alternators  are  seldom  made  now.  If  single-phase  service  is 
demanded  the  manufacturers  furnish  a  three-phase  machine 
instead  and  give  it  a  single-phase  rating  equal  to  about  70 
per  cent,  of  the  three-phase  rating.  The  single-phase  load  is 


166  ELECTRICAL  MACHINERY  [ART.  255 

then  carried  on  any  two  of  the  three  leads  of  the  three-phase 
generator.     See  "Three-phase  Alternator"  (Art.  258). 

255.  A  Table  of  Average  Performance  Values  for  Standard 
Two-  and  Three-phase  Generators  of  capacities  up  to  2,000 
kva.,  will  be  found  in  the  author's  AMERICAN  ELECTRICIANS' 
HANDBOOK.     In  this  table  are  given  the  current  values,  the 
efficiencies  at  various  loads  and  the  exciter  capacities  ordi- 
narily required  for  240,  480,  600,  1,200,  2,200  and  2,400-volt 
machines. 

256.  Performance    Guarantees    on     Alternating-current 
Generators  are,  in  the  United  States,  now   practically    al- 
ways made  on  the  continuous  rating  (A.  I.  E.  E.  Std.  Rule 
281)*  bases.     That  is,  the  generators  are  rated  at  the  kilo- 
volt-ampere  outputs  at  which  they  will  operate  continuously 
without  excessive   temperature   rise.     The   maximum   tem- 
perature rise  specified  is  usually  50  deg.  on  the  basis  of  a 
40-deg.  reference,  ambient  f  or  "room"  temperature.     These 
temperature  rises  are  based   on   operating  the  machines  on 
normal    excitation,    voltage   and   frequency   at   a   specified 
power-factor — usually   80  per  cent.     That  is,  the  "normal" 
method  of  rating  whereunder  an  overload — possibly  25  per 
cent,  for  two  hours — was  guaranteed,  is  no  longer  used  by  the 
leading  manufacturers  of  electrical  machinery.     The  above 
temperatures  are  all  based  on  the  thermometer  method  of 
observation. 

257.  Two-phase  Alternator. — In  a  generator  of  the  type 
indicated  in  Fig.  176  the  centers  of  the  two  component  coils 
/  and  II  are  90-  electrical  deg.  apart  and   the  single-phase 
electromotive  forces  generated  in  coils  I  and  //  by  the  pas- 
sage of  the  field  system  past  them,  differ  in  phase  by  90  deg. 
This  property  has  given  rise  to  the  term  quarter-phase  for 
this  type  of  machine,  but  it  is  more  frequently  called  a  two- 
phase  machine.     The  electromotive  force  in   coil   I  is   zero 
when  that  in  coil  //  is  a  maximum,   and  vice  versa.     The 

*"A  machine  rated  for  continuous  service  shall  be  able  to  operate  continuously 
at  its  rated  output  without  exceeding  any  of  the  limitations  referred  to  in  260." 

t  The  ambient  temperature  (A.  I.  E.  E.  Std.  Rule  No.  303)  is  the  temperature  of 
the  air  or  water,  which  in  coming  into  contact  with  the  heated  parts  of  a  machine, 
carries  off  its  heat.  See  also  Art.  476. 


SEC.  6]          ALTERNATING-CURRENT  GENERATORS 


167 


curves  of  electromotive  force  in  coils  /  and  II  may  be 
plotted  as  indicated  in  Fig.  177.  Fig.  178  shows  two 
methods  of  connecting  the  armature  windings  of  two-phase 
alternators.  The  armature  coils  can  be  arranged  in  one  or 


Curve  of  Current  in-one  Set  of  Coifs  ^ 
Gurve  of  Current  \ 

in  other  Set 
X     / 
,.eo-  ^27o« 


FIG.  176. — Diagram  for  two-phase  alternator. 

more  slots  per  pole  as  diagrammatically  suggested  in  Fig 
179.  In  commercial  machines  the  windings  are  almost 
always  arranged  in  more  than  one  slot  per  pole.  See  the 
author's  PRACTICAL  ELECTRIC- 
ITY for  further  information  in 
regard  to  two-phase  currents. 

258.  Three-phase  Alternator 
Coils  are  arranged  as  illustrated 
diagrammatically  by  coils  /,  // 
and  ///  of  Fig.  180,  and  the 
curves  of  instantaneous  electromotive  force  are  displaced 
from  one  another  by  60  deg.  as  indicated  in  Fig.  181.  This 
arrangement  of  coils  is  really  a  six-phase  grouping,  and  in 
connecting  the  winding  for  three-phase,  the  coils  of  one  of 
the  phases  must  be  connected  in  the  reverse  sense  from  the 


FIG.  177. — Graphs  of  two-phase 
current. 


168 


ELECTRICAL  MACHINERY 


[ART.  259 


other  two.      This  will  give  the  true  three-phase  arrangement 
in  which  the  e.m.f.  curves  are  as  in  Fig.  182.     These  curves 

One  Sef of  Armature  Coils 
\\    OfherSei-do.  External  Circuit 


-  Four -Wire  System 

Three -Wire  System. 

FIG.  178. — Methods    of    connecting    two-phase    generator    armature 

windings. 

also  represent  the  e.m.f s.  for  the  winding  in  Fig.  183  with 
the  three  phases  connected  in  the  same  sense.     Here  three 


•Stationary        , 
Armature'*' 

..'RtvoMrtg 


One  Slot  per  Pole.  Two  Slots  per  Pole, 

FIG.  179. — Two-phase  generator  armature  windings. 

coils  are  distributed  over  a  double-pole  pitch,  and  the  phase 
displacement  between  the  e.m.fs.  is  120  deg. 


FIG.  180.— Six-phi 
grouping. 


FIG.  181. — Graphs  of  instanta- 
neous electromotive  forces. 


259.    The    Two    Methods    of    Connecting    Three-phase 
Armature  Windings  are  shown  in  Fig.  184.     These  methods 


SEC.  6]         ALTERNATING-CURRENT  GENERATORS 


169 


are  discussed  in  more  detail  in  the  author's  PRACTICAL 
ELECTRICITY.  Armature  windings  can  be  arranged  in  one 
or  more  slots  per  pole  (Fig.  185).  The  Y  method  of  con- 
nection is  almost  invariably 
used  for  modern  three-phase 
generators. 

260.  Exciters  for  Alternat- 
ing-current Generators  are 
usually  compound-wound,  flat- 
compounded,  and  rated  at  125 
or  250  volts.  It  is  especially 
desirable  that  they  be  "stable," 
if  direct-connected  to  the  shaft 

of  an  alternator.  By  a  stable  generator  is  meant  one  that  does 
not  have  an  excessive  rise  or  fall  in  terminal  voltage  with  a 
corresponding  change  in  speed.  Standard  direct-current  ma- 


FIG.  182. — Curves  of  three-phase 
currents. 


Stationary 
Armature 


FIG.  183. — Diagram  for  three-phase,  Y-connected  alternator. 

chines  of  the  desired  rating  are  used  where  the  exciters  are 
separately  driven.  Separately-driven  exciters  are  usually  pref- 
erable for  most  applications  on  account  of  the  fact  that  the 


170 


ELECTRICAL  MACHINERY 


[ART.  261 


system  is  thereby  rendered  much  more  flexible ;  any  drop  in  the 
speed  of  the  alternator  does  not  cause  a  corresponding  drop  in 
the  exciter  voltage,  and  the  regulation  of  the  plant  as  a  whole 
is  improved.  Furthermore,  if  the  exciter  is  not  direct-con- 
nected, an  accident  to  it  will  not  necessitate  shutting  down 
the  generator,  assuming  that  there  is  a  duplicate  exciter  set. 


External  Circuit. 
Common  \ 

Connection  '; 


Delta  (A)  Connection  Y  Connection 

FIG.  184. — Methods  of  connecting  three-phase  armature  coils. 

261.  It  Is  Necessary  That  the  Exciter  Capacity  be  Ample 
to  Provide  Reserve  Capacity. — To  make  the  exciter  plant 
as  reliable  as  possible,  storage  batteries  are  being  installed  in 
connection  with  the  exciting  generators  in  some  plants  in 
such  a  way  that  current  may  be  furnished  to  the  field  circuits 
of  the  alternators,  even  though  all  rotating  apparatus  be  at 


Two  Slots  per  Pok  One  Slot  p«r  Pol« 

FIG.  185. — Three-phase  armature  windings. 

a  standstill.  As  an  example  of  the  amount  of  reserve  capacity 
that  is  sometimes  installed:  in  the  first  power  plant  of  the 
Niagara  Falls  Power  Company  four  exciters  are  installed, 
each  one  having  sufficient  capacity  to  excite  the  entire  plant, 
and  each  driven  by  its  own  turbine,  fed  by  a  separate  penstock. 
262.  Exciter  Drives. — It  is  apparent  that  where  separately- 
driven  exciters  are  used  the  prime  movers  should  be  such 


SEC.  6]         ALTERNATING-CURRENT  GENERATORS 


171 


Vertical  Water-  Wheel 
Generator 


•Field  Frame 
•Exciter 


that  the  exciters  may  be  started  independently  of  the  energy 
furnished  by  the  alternators.  Steam-,  water-,  or  gas-driven 
units  are  necessary  unless  a  storage  battery  or  power  from  an 
external  source  is  available  for  excitation  of  the  plant  when 
first  starting  up.  With  the  bus-bars  excited,  motor-driven 
units  may  be  operated  and  they  are  preferable  in  many  cases. 

263.  Exciter  Capacity. — General  figures  for  the  capacity  of 
an  exciter  for  any  machine  run  from  2.5  per  cent,  of  the 
capacity  of  the  alternator,  for 

moderate  speeds  and  small 
sizes,  to  0.5  per  cent,  of  the  al- 
ternator capacity,  or  a  trifle 
less,  for  large,  highspeed,  tur- 
bine units.  Two  per  cent,  is 
a  value  commonly  used  in  the 
absence  of  definite  data.  This 
is  too  low  in  a  very  few  cases, 
but  more  often  in  error  on  the 
safe  side. 

264.  A   Belt-driven   Exciter 
for    a    Vertical     Waterwheel 
Alternator  is  shown  in  Fig.  173 
and  186.     Note  that  a  quarter- 
twist  belt  is  used  for  driving 
the  direct-current  machine.     A 
sectional  view  of  an  alternator 
of  the  type  shown  in  this  illus- 
tration is  given  in  Fig.  172. 

965     Piplrl       T>i<5rTiflrtrp       FlG-  186- — An  example  of  a  ver- 

,5  b  o .  *  i  e  i  a     .uiscnarge  tical  waterwheel  generator  instal- 
Switches  and  Resistors  should  lation. 
be  provided  for  automatically 

discharging  the  field  circuits  of  alternating-current  generators 
when  the  field  switch  is  opened.  If  such  provision  is  not 
made,  the  high  e.m.f.  of  self-induction  developed  when  the 
switch  is  opened,  is  liable  to  puncture  and  possibly  ground 
the  field  winding  of  the  machine.  See  Figs.  116  and  117  for 
information  relating  to  field-discharge  switches  and  resistors 
as  applied  to  direct-current  motors  and  generators. 


172  ELECTRICAL  MACHINERY  [ART.  266 

266.  Explanations  of  the  Derivations  of   the   Following 
Formulas  and  Rules  will  in  most  cases  be  found  in  the  author's 
PRACTICAL  ELECTRICITY.     To  explain  here  why  the  formulas 
given  are  correct  would  require  considerable  space  and  would 
in  effect  be  a  repetition  of  material  which  is  treated  rather 
fully  in  the  other  book  just  referred  to. 

267.  To  Compute  the  Horse-power  Required  to  Drive  an 
Alternating-current,  Single  -phase  Generator,  on  the  Basis  of 
its  Kilovolt-ampere  Output,  the  following  equation  may  be 
used: 

SOA\  t  kva.0  X  p.f.  ,, 

P'{  =:   E  X  0  746  (horse-power) 

,  h.p.i  X  E  X  0.746 

(25)  kva.0  =  -        -  —  (kilo  volt-amperes) 

/OA^  -        h.p.i  X  E  X  0.746 

(26)  p.f.  =  --  j—  (power-factor) 


(efficiency) 


Wherein,  h.p.i  =  the  horse-power  input  to  the  generator. 
kva.o  —  the  kilovolt-ampere  output  of  the  generator,  p.f.  = 
the  power-factor  of  the  load  which  the  generator  is  serving. 
E  =  the  efficiency  of  the  generator,  for  the  conditions  under 
consideration,  expressed  decimally. 

EXAMPLE.  —  What  horse-power  is  required  to  drive  a  single-phase  gen- 
erator which  is  delivering  625  kva.  at  80  per  cent,  power-factor,  it  being 
assumed  that  the  efficiency  of  the  machine  at  this  load  is  90  per  cent.? 
SOLUTION.  —  Substitute  in  equation  (24):  h.p.i  =  (kva.0  X  p.f.)  -%-  (E 
X  0.746)  =  (625  X  0,80)  ^  (0.90  X  0.746)  =  745  h.p. 

268.  To  Compute  for  an  Alternating-current  Single-phase 
Generator,  Either  the  Horse-power  required  to  Drive  It,  Its 
Voltage,  Current,  Efficiency,  Power  Factor,  When  the  Value 
for  Only  One  of  These  Quantities  is  Not  Known,  one  of  the 

following  formulas  may  be  used: 

(28)  h.p.t  =  E  *  *'  *P/-  (horse-power) 


SEC.  6]         ALTERNATING-CURRENT  GENERATORS 


173 


(29) 


(30) 


(31) 


(32) 


h.p.t  X  E  X  746 
/i  X  p.f. 


Ii  = 


p.f.  = 


E  = 


h.p.j  X  E  X  746 

E  X  p.f. 
h.p.j  X  E  X  746 


E 


EXh 

liX  p.f. 


(volts) 


(amperes) 
(power-factor) 


(efficiency) 


h.p.i  X  746 

Wherein,  h.p.i  =  the  power  input  to  the  generator,  in  horse- 
power. E  —  the  voltage  impressed  by  the  generator  on  the 
external  circuit.  Ii  =  the  current  circulated  in  the  external 
circuit  by  the  generator,  p.f.  =  the  power-factor  of  the  load 
on  the  external  circuit,  expressed  decimally.  E  =  efficiency  of 
the  generator,  expressed  decimally. 


Switchboard^ 


•Single -Phase 
Ben-Driven  Generator 
'Efficiency  at  this  Load 
is  84%  and  the  power 
factor  of  th&Load 
'    82%) 


FIG.  187. — What  power  in  the  generator  taking? 

EXAMPLE. — What  horse-power  would  be  required  to  drive  the  single- 
phase  generator  of  Fig.  187  with  the  conditions  as  there  specified,  it  being 
assumed  that  under  these  conditions  the  efficiency  of  the  machine  is  84 
per  cent.,  the  e.m.f.  impressed  on  the  external  circuit  is  600,  the  current 
is  144  amp.  and  the  power-factor  of  the  load  is  82  per  cent.?  SOLUTION. 
—Substitute  in  equation  (28) :  h.p.i  =  (E  X  /i  X  p./.)  -*•  (E  X  746)  = 
(600  X  144  X  0.82)  -T-  (0.84  X  746)  =  113  h.p.  That  is,  under  these 
conditions  the  engine  would  have  to  deliver  113  h.p.  at  the  generator 
pulley,  P. 

269.  To  Compute  the  Kilowatt  Output  of  Any  Alternating- 
current,  Single-phase  Generator,  this  equation  may  be  used : 

E  X  Ii  X  p.f. 


(33) 


kw.0  = 


1,000 


(kilowatts) 


174  ELECTRICAL  MACHINERY  [ART.  270 

w       fa£^X  1,000 

-F^r^r  (volts) 

T  kW'°  X   1>OOQ 

1  =       E  X  p.f.  (amperes) 

n  f  fc^oX    1,000 

p./.  =  -    ^       -  —  (power-factor) 

Wherein,  all  of  the  symbols  have  the  same  meanings  as  in 
the  above  Articles  except  that  kw.0  =  power  output  of  the  gen- 
erator in  kilowatts. 

270.  To  Compute  the  Kilovolt-ampere  Output,  the  Current 
or  the  Voltage  of  any  Alternating-current,  Single  -phase  Gen- 
erator one  of  the  following  formulas  may  be  used  : 

W  V  T 

(37)  kva.o  =    1  nrv/  (kilovolt-amperes) 

1, 


(38)  E  =  kva'°  *  (volts) 

ll 

,QOx  j        kva.0  X  1,000 

(39)  /i  =  -      —  =T—  (amperes) 

£L 

Wherein,  the  symbols  have  the  same  meanings  as  above. 

271.  To  Compute  the  Kilovolt-amperes  Output,  the  Current 
or  the  Voltage  of  Any  Three-phase,  Alternating-current  Gen- 
erator one  of  the  following  formulas  may  be  used  : 

(40)  kva.o  =  E  X  73  X  0.00173  (kilovolt-amperes) 


//I1N  _       kva.0  X  577 

(41)  E  =  -  —  y~  (volts) 

//10,  kva.o  X  577  , 

(42)  73  =  -     —  =  --  (amperes) 

hi 

Wherein,  kva.0  =  the  output  of  the  generator,  in  kilovolt-am- 
peres. E  =  the  e.m.f  .  impressed  by  the  machine  between  phase 
wires  on  the  external  circuit  in  volts.  1%  =  the  current,  in 
amperes,  in  each  of  the  three-phase  wires,  it  being  assumed 
for  all  of  this  notation  that  the  load  on  the  three  phases  is 
balanced. 

EXAMPLE.  —  What  is  the  kilovolt-ampere  output  of  the  three-phase, 


SEC.  6]         ALTERNATING-CURRENT  GENERATORS  175 

alternating-current  generator  of  Fig.  188  when  it  is  delivering,  as  shown, 
a  current  of  54  amp.  in  each  of  the  three-phase  wires  and  impressing  on 
the  external  circuit  a  voltage  of  2,200?  SOLUTION.  —  Substitute  in  equa- 
tion (40):  kva.<,  =  E  X  h  X  0.00173  =  2,200  X  54  X  0.00173  =  205.5 
kva. 

272.  To  Compute  for  a  Three-phase,  Alternating-current 
Generator,  Either  the  Horse-power  Required  to  Drive  It,  the 
Kilowatt-ampere  Output,  the  Power  Factor  or  the  Efficiency, 
When  the  Value  for  Only  One  of  these  Quantities  is  Not 
Known,  one  of  the  following  formulas  may  be  used: 

//40x  kva.o  X  p.f.  ^ 

(43)        h.p.i  =•-   E  x  0  (horse-power) 


h.p.i  X  E  X  0.746 

(44)        kva.o  =  -  —f—  (kilo  volt-amperes) 

PJ. 


/Voltmeters 
'Read  2200  Volts 


'"Three-Phase,  Alternating? 
Current  Generator 


FIG.  188. — Example    in    computing    Kva.     Output    of    a    three-phase 

generator. 

h.p.t  X  E  X  0.746 

(45)  P-f.=  -       —j—  (power-factor) 

Kva.o 

,,         kva.o  X  p.f.  f  ~,  . 

(46)  E  =  l~—^-^a  (efficiency) 


Wherein,  all  of  the  symbols  have  the  same  meanings  as  in  the 
above  articles  except  that  kva.0  =  the  kilovolt-ampere  output 
of  the  generator. 

EXAMPLE. — A  three-phase  generator  has  a  full-load  rating  of  600  kva. 
When  delivering  full-load  kilovolt-amperes  to  the  load  at  80  per  cent, 
power-factor,  what  horse-power  would  be  required  to  drive  this  machine, 
assuming  that  its  efficiency  is  90  per  cent.?  SOLUTION. — Substitute  in 
equation  (43):  h.p.i  =  (kva.0  X  p./.)  -5-  (E  X  0.746)  =  (600  X  0.80)  + 


176  ELECTRICAL  MACHINERY  [ART.  273 

(0.90  X  0.746)  =  715  h.p.     Therefore,  under  these  conditions  it  would 
require  715  h.p.  to  drive  the  generator. 

273.  To  Compute  for  Any  Alternating-current  Generator, 
Either  the  Kilowatt  Output,  the  Voltage,  the  Current  or  the 
Power  Factor,  When  the  Value  for  Only  One  of  These  Quan- 
tities is  Not  Known,  one  of  the  following  equations  may  be 
used: 

(47)  kw.0  =  E  X  /3  X  p.f.  X  0.00173  (kilowatts) 

(48)  ^=^><J77  (voltg) 

^3   A   p.J. 

kw.0  X  577  , 

(49)  J3  =  (amperes) 


(50)  p.f.  =  rjp  (power-factor) 

Wherein,  all  of  the  symbols  have  the  same  meanings  as  speci- 
fied above  except  that  kw.0  =  the  output  of  the  generator  in 
kilowatts. 

274.  To  Compute  for  Any  Three-phase  Generator,  the 
Horse-power  Required  to  Drive  It,  Its  Voltage,  Current, 
Efficiency  or  Power  Factor,  When  the  Value  of  Only  One  of 
These  Quantities  is  Not  Known,  one  of  the  following  formulas 
may  (it  being  assumed  that  the  electrical  load  is  equally 
balanced  between  the  three  phases)  be  used: 

(51)  A.p.«  =  V  (horse-power) 

(52)  E  m  ^••• 

(53)  l3  =  h.p.i 


P'f'  =  EXI  (power-factor) 

=  E  X  IB  X  p.f. 
h.p.i  X  430.7 

Wherein,  h.p.i  —  the  power  input  to  the  generator,  in  horse- 


SEC.  6]         ALTERNATING-CURRENT  GENERATORS 


177 


power.  E  =  the  e.m.f.  in  volts  impressed  by  the  machine  be- 
tween phase  wires  on  the  external  circuit.  Iz  =  the  current 
circulated  by  the  machine  in  each  of  the  three-phase  wires. 
p.f.  =  power-factor  of  the  load  served  by  the  generator.  E  = 
efficiency  of  the  machine,  expressed  decimally,  for  the  condi- 
tions under  consideration. 

EXAMPLE. — What  horse-power  input  would  be  required  at  the  pulley, 
P,  of  the  three-phase  generator  shown  in  Fig.  189  when  the  machine  is 
impressing  2,200  volts  on  the  external  circuit  and  circulating  a  current 
of  23  amp.,  it  being  assumed  that  the  power-factor  of  the  load  is  85  per 
cent,  and  that  the  efficiency  of  the  generator  is  90  per  cent.?  SOLUTION. 
—Substitute  in  equation  (51):  fc.p.<  =  (E  X  73  X  p./.)  -*•  (E  X  430.7)  = 
(2,200  X  23  X  0.85)  -J-  (0.90  X  430.7  =  111  h.p. 


Ammeters  Read  23  Amp. 


FIG.  189. — Size  engine  required  to  drive  a  three-phase  alternating-cur- 
rent generator. 

275.  To  Find  the  Size  Engine  Required  to  Drive  an  Alter- 
nating-current Generator  the  same  general  procedure  may  be 
followed  as  that  used  in  ascertaining  the  engine  capacity  for 
direct-current  machines,  which  is  described  in  Art.  51.  How- 
ever, with  an  alternating-current  generator  it  must  be  re- 
membered that  the  kilo  volt-ampere  rating  of  the  machine  must 
be  multiplied  by  the  power-factor  of  the  load  which  will  be 
served  in  order  to  ascertain  the  power  load  in  kilowatts.  One 
of  the  preceding  equations  can  be  used  in  determining  the 
horse-power  necessary  at  the  generator  pulley  or  shaft  to  pull 
a  given  electrical  load.  In  the  author's  AMERICAN  ELEC- 
TRICIANS' HANDBOOK  are  given  tables  showing  the  actual  ef- 
ficiencies of  commercial  alternating-current  generators.  It 
should  be  noted  (see  Art.  253)  that  with  alternating-current 
generators  the  speed  is  determined  by  the  number  of  poles  on 
the  generator  and  by  the.  frequency  which  it  is  desired  to 
produce. 
12 


SECTION  7 

MANAGEMENT    OF    ALTERNATING-CURRENT 
GENERATORS 

276.  In  the  Management  of  Alternating-current  Generators 

many  of  the  principles  already  discussed  in  Sec.  2,  "  Manage- 
ment of  Direct-current  Generators,"  apply.  In  general,  the 
alternators  should  receive  the  same  care  in  regard  to  oiling 
(Art.  76),  cleanliness  and  general  attention  as  do  direct-cur- 
rent dynamos.  The  reader  is  advised  to  review  Sec.  2. 

277.  Synchronizing.* — Two    or    more    alternating-current 
generators  will  not  operate  in  parallel  unless  (1)  their  voltages, 
as  registered  by  a  voltmeter,  are  the  same;  (2)  their  frequencies 
are  the  same;  and  (3)  their  voltages  in  phase.     If  the  machines 
are  not  in  phase,  even  if  their  indicated  voltages  and  their 
frequencies  are  the  same  the  voltage  of  one  will,  at  given  in- 
stants, be  different  from  that  of  the  other  and  there  will  be 
an  interchange  of  current  between  the  machines.     When  two 
or  more  generators  all  satisfy  the  three  above  requirements 
they  are  "in  step"  or  in  synchronism.     Synchronizing  is  the 
operation  of  getting  machines  into  synchronism.     Incandes- 
cent lamps  or  instruments  are,  as  described  in  other  para- 
graphs, used  for  indicating  when  machines  are  in  synchronism. 

278.  Synchronizing  a  Single-phase  Circuit  with  Lamps.— 
The  Elementary  Principle  involved  in  determining  synchronism 
is  indicated  in  Fig.  190.     If  the  voltage  and  frequency  of  gen- 
erators A  and  B  are  the  same  and  the  machines  are  in  phase, 
point  a  will  be  at  the  same  potential  at  every  instant  as  will 
point  a'.     Hence  the  lamps  between  a  and  a'  will  not  light  so 
long  as  the  three  conditions  are  satisfied.     So  long  as  the 

•  For  a  complete  discussion  of  the  various  methods,  and  for  diagrams  of  all  synchron- 
izing circuits  in  common  use  for  both  lamps  and  synchroscopes  see  ELECTRIC  JOURNAL, 
articles  by.  Harold  Brown,  May,  1912,  and  July,  1912.  The  material  on  this  subject 
herein  is  largely  from  these  articles. 

178 


SEC.  7]         ALTERNATING-CURRENT  GENERATORS 


179 


conditions  are  not  satisfied  there  will  be  a  fluctuating  cross- 
current from  a  to  a'  and  a  constant  fluctuating  of  the  brilliancy 
of  the  incandescent  lamps.  When  the  lamps  become  dark  and 
remain  so,  the  generators  are  in  synchronism  and  may  be 
thrown  together.  Had  the  connection  at  a'  been  made  to  the 
6'  generator  lead,  the  lamps  would  be  bright  when  the  genera- 
tors were  in  synchronism,  but  for  reasons  outlined  in  another 
paragraph  the  connection  shown  which  provides  the  "dark 
lamp"  method  of  synchronizing  is  preferred.  The  second 
pair  of  lamps  between  b  and  &'  is  provided  to  insure  against 
accident  in  case  the  a-af  set  were  broken.  The  same  condi- 
tions occur  in  the  a— a'  set  as  in  the  b— b'  set.  A  voltmeter  of 


Bus  -  Bars 

( 

1 

s     ,.  Main  Switches.     , 

_L 

Incandescent 

Q 

Lamps 

n1 

b 

o   o 

t) 

-  Generators-... 


FIG.  190. — Circuits  for  synchro- 
nizing with  lamps. 


FIG.  191. — Circuits  for  syn- 
chronizing high-voltage  circuits 
with  lamps. 


proper  rating  can  be  substituted  for  the  lamps.  Where  the 
voltage  generated  is  so  high  that  it  is  not  desirable  to  connect 
a  sufficient  number  of  lamps  in  series  for  it,  a  single  pair  of 
lamps  fed  through  voltage  transformers  can  be  used  for  syn- 
chronizing, is  suggested  in  Fig.  191. 

279.  Phasing  Out. — Prior  to  connecting  the  leads  from  a 
polyphase  generator  (which  is  to  operate  in  parallel  with 
others)  to  the  generator  switch,  the  circuits  must  be  "phased 
out."  That  is,  the  leads  must  be  so  arranged  that  each  lead 
from  the  generator  will,  when  the  generator  switch  is  thrown, 
connect  to  the  corresponding  lead  of  the  other  generator.  If 
this  is  not  arranged  there  may  be  considerable  damage  done 
due  to  an  interchange  of  current  when  the  two  machines  are 


180 


ELECTRICAL  MACHINERY 


[ART.  280 


paralleled.  After  once  phasing  out  it  is  necessary  to  syn- 
chronize but  one  phase  of  the  machine  with  the  corresponding 
phase  of  the  other  machine. 

280.  Connections  for  Phasing  Out  Three-phase  Circuits  are 
shown  in  Fig.  192.  If  voltage  transformers  are  not  used  the 
sum  of  the  voltages  of  the  lamps  in  each  line  should  be  ap- 
proximately the  same  as  the  voltage  of  the  circuits.  On  440- 
volt  circuits,  two  220- volt  or  four  110- volt  lamps  should  be 
used  in  each  phasing-out  lead.  To  phase  out,  run  the  two 
machines  at  about  synchronous  speed.  If  the  lamps  do  not 


Voltage  Transformer 


With  Voltage 
Transformers.' 


Without  Voltage 
Transformers.. 


FIG.  192. — Connections  for  phasing  out  three-phase  circuits. 

all  become  bright  and  dark  together,  interchange  any  two 
of  the  main  leads  on  one  side  of  the  switch,  leaving  the  lamps 
connected  to  the  same  switch  terminals,  after  which  the  lamps 
should  all  fluctuate  together,  indicating  that  the  connections 
are  correct.  The  machines  are  in  phase  when  all  the  lamps 
are  dark. 

281.  The  Synchronizing  Connections  for  More  Than  Two 
Three-phase  Generators  are  shown  in  Fig.  193  although  only 
two  machines  are  illustrated  in  this  diagram.  A  synchroniz- 
ing plug  may  be  used  instead  of  the  single-pole  synchronizing 
switch  shown.  The  illustration  indicates  the  connections  used 
where  machines  are  to  be  synchronized  to  a  bus.  *  Where  only 
two  machines  are  to  be  synchronized,  the  connections  are  the 
same  as  shown  in  Fig.  193  except  that  the  bus  transformer 


SEC.  7]         ALTERNATING-CURRENT  GENERATORS 


181 


and  the  corresponding  lamp  are  omitted  and  one  plug  or  syn- 
chronizing switch  is  required  instead  of  two. 

282.  Synchronizing  Dark  or  Light. — Synchronizing  dark 
appears  to  be  the  preferable  method.  All  the  connections 
shown  in  this  book  are  for  "synchronizing  dark."  When  the 
lamps  are  "dark"  the  machines  are  in  phase  and  it  is  neces- 
sary to  close  the  switch  when  the  variation  in  light  is  the 
slowest  obtainable  or  ceases  altogether,  that  is,  at  or  just  be- 
fore the  middle  of  the  longest  dark  period.  Should  a  filament 
break,  the  synchronizing  lamps  would  remain  dark  and  thus 
apparently  indicate  synchronism  and  possibly  cause  an  acci- 
dent. Therefore,  it  is  considered  desirable  by  some  to  reverse 


Bus -Bars 


Lamp 


Bus  Transformer 


•Synchronizing 


Generators- 


FIG.  193. — Connections  for  synchronizing  three-phase   circuits  where 
transformers  are  required. 

the  synchronizing  circuit  connections  and  thereby  synchronize 
"light."  Synchronizing  light  eliminates  the  danger  due  to 
the  breaking  of  a  filament,  but  has  the  disadvantage  that  the 
time  of  greatest  brilliancy  is  difficult  of  determination.  The 
"light"  period  is  relatively  long  compared  with  the  dark 
period,  so  that  synchronizing  light  is  usually  considered  the 
more  difficult  and  were  it  not  that  with  the  "synchronizing 
light"  method  the  danger  due  to  filament  breakage  is  elimi- 
nated, the  method  would  never  be  used.  The  probability  of 
a  filament  breaking  justat  the  time  of  approaching  synchronism 
and  when  the  machines  are  not  in  phase  is  remote.  If  it 
occurs  at  any  other  time  in  the  operation  it  will  be  noticed. 
As  a  protection  against  accidents  due  to  such  breakage,  two 
synchronizing  lamps  should  always  be  placed  in  multiple. 


182  ELECTRICAL  MACHINERY  [ART.  283 

283.  The  Number  of  Lamps  to  Use  in  a  Group  to  Indicate 
Synchronism  is  determined  by  the  voltage  of  the  generators. 
With  high-voltage  circuits  it  is  not  feasible  to  use  a  sufficient 
number  of  lamps,  so  a  transformer  is  then  employed  (Fig.  193) 
that  has  a  voltage  sufficient  for  a  110-volt  lamp.     See  the 
diagrams.     The  greatest  voltage  impressed  on  the  lamps  is 
double  that  of  the  voltage  transformers  or  generators.     Thus 
the  maximum  voltage  on  the  lamps  where  two  220-volt  gen- 
erators are  being  synchronized  is  440  volts.     The  dark  period 
may  be  shortened  by  impressing  a  voltage  higher  than  their 
normal  on  the  lamps.     For  two  220-volt  machines,  for  example, 
three  110-volt  lamps  might  be  used. 

284.  Synchroscopes  are  instruments  that  indicate  the  dif- 
ference in  phase  between  two  electromotive  forces  at  every 
instant.     They  show  whether  the  machine  to  be  synchronized 
is  running  fast  or  slow  and  indicate  the  exact  instant  when 
the  machines  are  in  synchronism.     The  companies  which  man- 
ufacture  the   instruments   furnish   literature    describing  the 
theory  involved  and  which  gives  complete  circuit  diagrams. 

285.  While  for  Successful  Parallel  Operation,  it  is  not  neces- 
sary that  alternating-current  generators  (Fig.  194)  be  of  the 
same  type,  output,  and  speed,  it  is  universally  conceded  that 
the  question  of  wave  shape  or  form  is  important,  since  if  the 
waves  are  of  different  shapes,  cross-currents  will  always  be 
present.     Similar  wave  shapes  are  more  readily  obtained  with 
machines  of  similar  type.     Satisfactory  parallel  operation,  the 
previously  mentioned  conditions  being  fulfilled,  consist  in  ob- 
taining: (1)  Correct  division  of  the  load  among  the  machines; 
and  (2)  Freedom  from  hunting. 

286.  Division  of  Load. — Machines  with  similar  character- 
istics tend  to  divide  the  cpmmon  load  uniformly.     Such  a  pro- 
portional load  division  may  be  disturbed  if  the  steam  supply 
to  the  engines  is  defective  or  variable  from  any  cause.     The 
steam  supply  is  regulated  by  the  engine  governors,  and  defects 
in  one  or  more  of  these  governors  will  give  rise  to  poor  load 
division.     It  is  essential  that  the  governors  of  all  the  engines 
shall  have  similar  speed-regulation  characteristics  so  that  a 
sudden  change  in  the  load  shall  cause  the  same  amount  of 


SEC.  7]         ALTERNATING-CURRENT  GENERATORS 


183 


regulation  on  each  engine.  Correct  load  division  is,  therefore, 
essentially  a  problem  of  engine  governor  design.  It  is  some- 
times arranged  to  govern  all  the  engines  from  a  common 
throttle  valve,  but  this  plan  is  not  often  employed.  A  more 
usual  plan  consists  in  running  all  the  machines  except  one,  with 
their  stop  valves  full  open  and  their  governors  fixed,  so  that 
the  remaining  engine  may  take  up  any  variations  in  the  com- 
mon load. 

2300  Volt  60  Cycle  Bus 


Oil  Switches- 


400  kw. 

Belted 

Unit 

(Steam) 


•  f Alternating 

if'rf — ».    Oi/rrent 

•  Generators-. 


400  kw. 
Belted 

Unit 
(Steam) 


FIG.  194. — Internal-combustion  engine-driven  alternator,  operating  in 
parallel  with  steam-engine  driven  machine,  Shreveport,  La.,  plant  of  the 
Southwestern  Gas  and  Electric  Company.  (ELECTRIC  WORLD,  July  4, 
1914,  page  37.) 

287.  Varying  the  Voltage  of  an  Alternator  Running  in  Par- 
allel With  Others  by  Adjusting  Its  Field  Rheostat  Will  Not 
Vary  the  Load  on  It  as  With  a  Direct-current  Generator. — 

To  increase  the  energy  delivered  by  an  alternator  it  is  neces- 
sary that  the  prime  mover  be  caused  to  do  more  work.  An 
engine  should  be  given  more  steam  or  a  waterwheel  more 
water. 

288.  Adjustment  of  Field  Current. — When  the  rheostats  of 
two  alternators,  running  in  parallel  at  normal  speed,  are  not 


184 


ELECTRICAL  MACHINERY 


[ART.  287 


adjusted  to  give  a  proper  excitation,  a  cross-current  will  flow 
between  the  armatures.  The  intensity  of  this  current  depends 
only  upon  the  difference  in  field  excitation  of  the  machines. 
It  may  vary  over  a  wide  range,  from  a  minimum  of  zero  when 
both  field  excitations  are  normal,  to  more  than  full-load  cur- 
rent when  they  differ  greatly.  The  effect  of  this  cross-current 
is  to  increase  the  temperature  of  the  armatures  and,  conse- 


Shunt  Field 


FIG.  195. — Two  three-phase  alternators  of  similar  characteristics  operat- 
ing in  parallel. 

quently,  to  decrease  the  output  of  the  generators.  It  is  im- 
portant that  the  rheostats  be  so  adjusted  as  to  reduce  it  to  a 
minimum.  This  cross-current  registers  on  the  ammeters  of 
both  generators  and  usually  increases  both  readings.  The 
sum  of  the  ammeter  readings  will  be  a  minimum  when  the 
idle  or  cross-current  is  zero. 

In  general,  the  proper  field  current  for  a  machine  running 
in  parallel  with  others  is  that  which  it  would  have  if  running 
alone  and  delivering  its  load  at  the  same  voltage.  In  order 
to  determine  the  proper  position  of  the  rheostats  it  is  neces- 


SEC.  7]         ALTERNATING-CURRENT  GENERATORS  185 

sary  to  make  trial  adjustments  after  the  alternators  are  par- 
alleled, until  that  position  is  found  at  which  the  sum  of  the 
alternating-current  ammeter  readings  is  a  minimum. 

EXAMPLE. — To  illustrate  this  method  consider  two  similar  alternators, 
A  and  B,  Fig.  195,  operating  in  parallel.  When  the  generator  field 
rheostats  of  both  are  properly  adjusted  no  cross-currents  will  flow  through 
the  armatures  and  the  main  ammeters  (A,  A,  and  A)  will  show  equal 
readings  if  each  machine  is  receiving  the  same  amount  of  power  from  its 
prime  mover.  If  the  rheostat  of  A  be  partly  cut  in  so  as  to  reduce  its- 
field  current,  a  cross-current,  lagging  in  B  and  leading  in  A,  will  flow 
between  the  armatures,  the  effect  of  which  will  be  to  strengthen  A's 
magnetization  and  weaken  B's  until  they  are  approximately  equal.  The 
resultant  e.m.f.  of  the  system  will  thereby  be  lowered. 

On  the  other  hand,  if  the  rheostat  of  B  be  partly  cut  out  so  as  to  in- 
crease its  field  current,  a  cross-current  leading  in  A  and  lagging  in  B 
will  flow  between  the  armatures,  strengthening  A's  magnetization  and 
weakening  B's  magnetization  until  they  are  again  equal.  The  resultant 
e.m.f.  of  the  system  will  thereby  be  raised.  A  cross-current  of  the  same 
character  is,  therefore,  produced  by  decreasing  one  field  current  or  in- 
creasing the  other,  i.e.,  in  both  cases  it  will  lead  in  the  first  machine  and 
lag  in  the  second  machine.  The  e.m.f.  of  the  system  will,  however,  be 
decreased  in  one  case  and  increased  in  the  other. 

It  is  obvious  that  by  simultaneously  adjusting  the  two  rheostats,  the 
strength  of  the  cross-current  may  be  varied  considerably  and  the  e.m.f. 
of  the  system  maintained  constant. 

For  the  first  trial  adjustment  cut  in  A's  rheostat  several  notches  and 
cut  out  B's  the  same  amount,  so  as  not  to  vary  the  e.m.f.  of  the  system. 
If  this  reduces  the  sum  of  the  main  ammeter  readings,  continue  the 
adjustment  in  the  same  direction  until  the  result  is  a  minimum.  After 
this  point  is  reached  a  further  adjustment  of  the  rheostat  in  either 
direction  will  increase  the  ammeter  readings.  If  the  first  adjustment 
increases  the  sum  of  the  ammeter  readings  it  is  being  made  in  the  wrong 
direction,  in  which  case  move  the  rheostats  back  to  the  original  positions 
and  then  cut  out  A's  rheostat  and  cut  in  B's.  If  both  adjustments 
increase  the  sum  of  the  ammeter  readings  the  original  positions  of  the 
rheostats  are  the  proper  ones. 

In  making  these  adjustments  of  the  rheostats  it  may  be 
found  difficult  to  locate  the  exact  points  at  which  the  cross- 
current is  a  minimum,  as  it  may  be  possible  to  move  the  rheo- 
stats over  a  considerable  range  when  near  the  correct  positions 
without  materially  changing  the  ammeter  readings.  When 
the  adjustment  is  carried  this  far,  it  is  close  enough  for  prac- 


186  ELECTRICAL  MACHINERY  [ART.  289 

tical  operation.  If  the  generators  are  provided  with  power- 
factor  meters,  the  same  result  may  be  obtained  by  adjusting 
all  these  to  read  the  same. 

289.  Hunting*  is  a  term  employed  to  describe  the  oscil- 
lations of  the  revolving  masses  of  the  machines  when  they  are 
accelerated  and  retarded  above  and  below  the  normal  average 
speed.     If  this  hunting  or  swinging  be  allowed  to  exceed  a 
certain  amount,  the  regulation  of  the  machines  becomes  un- 
stable and  they  may  break  out  of  step.     Freedom  from  cumu- 
lative hunting  is  consequently  essential.     The  swinging  action 
is  set  up  primarily  by  variations  in  the  rotative  speed  resulting 
from  irregularity  in  the  turning  force.     A  perfectly  uniform 
turning  moment  or  turning  force  cannot  be  obtained  with 
reciprocating  engines.     The  irregularity  in  the  turning  moment 
during  a  revolution  results  from  the  following  causes:   (1) 
Defective  distribution  of  steam  in  cylinders.     (2)  Short  con- 
necting rod.     (3)  Inertia  of  moving  parts. 

If  one  of  two  machines  running  in  parallel  momentarily  lags 
behind  the  other,  its  armature  receives  a  current  which  tends 
to  pull  the  machine  into  phase  and  accelerate  it  so  that  at 
the  instant  it  reaches  the  correct  phase  position  its  speed  is  a 
little  greater  than  that  of  the  other  machine,  which  is  now  in 
turn  accelerated.  The  machines  are  now  alternately  lagging 
and  leading  with  relation  to  one  another.  In  other  words, 
hunting  is  set  up. 

Whichever  engine  is,  for  the  instant,  accelerating,  will  have 
its  steam  supply  cut  down  by  the  governor.  If  the  governor 
is  too  sensitive,  it  will  over-govern,  cutting  down  the  steam 
and  the  speed  too  far.  An  instant  later,  the  over-governing 
will  be  in  the  opposite  sense,  and  this  process  will  repeat  itself. 
Similar  occurrences  will  simultaneously  be  taking  place  on  the 
other  engine,  and  thus  we  have  a  case  of  hunting  governors. 
By  this  hunting,  the  steam  supply  is  rendered  periodic  and 
varies  between  two  limits. 

290.  Surging  is  the  term  used  to  designate  the  current 
variations  during  hunting.     The  case  above  described  is  an 
instance  of  hunting  in  the  governors  due  to  change  of  load  and 

•STANDARD  HANDBOOK. 


SEC.  7]         ALTERNATING-CURRENT  GENERATORS  187 

to  over-sensitiveness  of  the  governors.  If,  however,  the  gov- 
ernors are  sluggish,  a  time  interval  elapses  between  an  acci- 
dental acceleration  and  its  correction  by  the  governor.  This 
lag  will,  in  response,  tend  to  set  up  hunting. 

291.  Prevention  of   Hunting.-— The  variations  in  turning 
moment  and  angular  speed  may  be  greatly  reduced  by  the 
use  of  a  heavy  flywheel,  as  this  tends  to  keep  the  rate  of 
revolution  uniform  by  virtue  of  storing  energy  and  giving  it 
out  again  during  the  course  of  each  revolution.     The  flywheel, 
however,  must  not  have  too  great  a  moment  (that  is,  it  must 
not  be  too  big)  as  it  adds  to  the  inertia  of  the  moving  parts 
and  may  prolong  hunting  if  once  started.     Hunting  may  some- 
times be  overcome  by  damping  the  governor  so  that  it  shall 
not  respond  to  small  and  quick  variations  in  speed  such  as 
occur  during  one  revolution,  but  shall  only  respond  to  steady 
and  continued  changes  in  speed.     This  result  is  obtained  by 
fitting  each  governor  with  a  suitable  dash  pot  so  that  it  is 
rendered  more  sluggish  and  will  make  no  alteration  in  the 
steam  supply  except  when  the  force  acting  on  the  governor 
is  continued  for  some  length  of  time. 

292.  Liability  to  Hunt  May  Sometimes  Be  Prevented  by 
Synchronizing  the   Engines   so  that  the  cranks  on  all  the 
engines  are  in  step,  and  the  variations  in  turning  moment  are 
coincident  in  all  the  engines.     This  plan  is  sometimes  effec- 
tive,  so  far  as  the  prevention  of  hunting  in  the  generating 
station  is  concerned,  but  it  cannot  always  be  utilized  owing 
to  the  time  taken  to  get  the  cranks  in  step,  especially  as  an 
engine  must  be  run  up  in  a  few  minutes  when  the  load  is 
coming  on  quickly.     It  also  is  apt  to  intensify  the  hunting  of 
the  apparatus  in  distant  substations.     With  steam  turbine- 
driven  generators,  this  hunting  difficulty  is  rare — practically 
unknown — and  the  use  of  high  and  uniform  speeds  facilitates 
the  problem  of  parallel  running. 

293.  The  Tendency  of  Generators  to  Hunt  May  Be  Mini- 
mized by  fitting  the  pole-pieces  of  the  field  magnets  with 
copper  bands  or  " dampers"  (see  Fig.  240)  in  which  eddy  cur- 
rents are  induced  by  the  shifting  and  distortion  of  the  field. 
These  currents  react  on  the  field  and  oppose  the  shifting  and 


188  ELECTRICAL  MACHINERY  [ART.  294 

thus  damp  the  oscillations.  A  suitable  construction  consists 
of  a  grid  of  copper  embedded  in  the  pole  face .  It  is  very  sel dom 
necessary  to  provide  such  "dampers"  on  pole-pieces  of  genera- 
tors for  modern  steam-engine  or  waterwheel  drive.  They 
are  usually  necessary  for  internal-combustion  engine-driven 
generators. 

294.  To  Start  a  Single  Alternator.* — (1)  See  that  there  is 
plenty  of  oil  in  the  bearings  and  that  the  oil  rings  are  free  to 
turn  and  that  all  switches  are  open.     (2)  Start  exciter  and 
adjust  for  normal  voltage.     Start  generator  slowly.     See  that 
the  oil  rings  are  turning.     (3)  Permit  the  machine  to  reach 
normal  speed.     Turn  the  generator  field  rheostat  so  that  all 
of  its  resistance  is  in  the  field  circuit.     Close  the  field  switch. 
(4)  Adjust  the  rheostat  of  the  exciter  for  the  normal  exciting 
voltage.     Slowly  increase  the  alternator  voltage  to  normal  by 
cutting  out  the  resistance  of  the  field  rheostat.     (5)  Close  the 
main  switch. 

295.  To  Start  an  Alternator  to  Run  in  Parallel  with  Others. 
— (1)  Bring  the  exciter  and  generator  to  speed  as  described  in 
the  above  paragraph.     Adjust  the  exciter  voltage  and  close 
the  field  switch,  the  generator  field  resistance  being  all  in.     (2) 
Adjust  the  generator  field  resistance  so  that  the  generator 
voltage  will  be  the  same  as  the  bus-bar  voltage.     (3)  Syn- 
chronize, as  outlined  in  Art.  277.     Close  the  main  switch. 
(4)  Adjust  the  field  rheostat  until  cross-currents  (Art.  288) 
are  a  minimum  and  adjust  the  governors  of  the  prime  movers 
so  that  the  load  will  be  properly  distributed  between  the  oper- 
ating units  in  proportion  to  their  capacities. 

296.  To  Cut  Out  a  Generator  Which  is  Running  in  Parallel 
with  Others.* — (1)  Preferably  cut  down  the  driving  power 
until  it  is  just  sufficient  to  run  the  generator  with  no-load. 
This  will  reduce  the  load  on  the  generator.     (2)  Adjust  the 
resistance  in  the  field  circuit  until  the  armature  current  is  a 
minimum.     (3)  Open  the  main  switch.     It  is  usually  sufficient, 
however,  to  simply  disconnect  the  machine  from  the  bus- 
bars, thereby  throwing  all  the  load  on  the  remaining  machine 

*WESTINaHOUSE  INSTRUCTION  BOOK. 


SEC.  7]         ALTERNATING-CURRENT  GENERATORS  189 

without  having  made  any  previous  adjustment  of  the  load  or 
of  the  field  current.  CAUTION. — The  field  circuit  of  a  generator 
to  be  disconnected  from  the  bus-bars  must  not  be  opened  before 
the  main  switch  has  been  opened;  for,  if  the  field  circuit  be 
opened  first,  a  heavy  current  will  flow  between  the  armatures. 


SECTION  8 

PRINCIPLES,  CONSTRUCTION  AND  CHARACTERISTICS 
OF  INDUCTION  AND  REPULSION  MOTORS 

297.  The  Principle  of  Operation  of  the  Induction  Motor  is 
illustrated  in  Fig.  196,  which  indicates  diagrammatically  a 
two-phase  revolving-field  generator  and  a  two-phase  induction 
motor  having  a  rotor  that  is  simply  a  bar  of  iron.  Thejn^.-j 
duction  motor  depends  for  its  operation  on  a  rotating  magnetic 
field.  There  is  no  electrical  connection  between  the  revolving 
and  stationary  parts  of  an  induction  motor.  Windings  of  the 
types  shown  in  the  illustration  are  not  used  in  commercial 
machines,  but  the  general  theory  involved  is  the  same  as  with 
commercial  windings.  The  revolving  field  (see  illustration)  of 


Revolving 
Field* 


Oerieralor 


Motor 


Rotor 


m. 

First  Position.         Second  Position.      Third  Position. 
FIG.  196. — Illustrating  the  principle  of  the  induction  motor. 

the  generator,  in  turning  in  the  direction  shown  by  the  arrow, 
generates  a  two-phase  current  which  is  transmitted  to  the 
motor.  For  a  more  extended  explanation  of  the  operating 
principles  of  induction  motors  see  the  author's  PRACTICAL 
ELECTRICITY.  What  then  occurs  may  be  explained  thus: 

EXPLANATION. — The  current,  in  conductors  of  one  phase,  magnetizes 
poles  A  and  B  and  that  in  the  other  phase  the  poles  C  and  D.  The  wind- 
ing is  so  arranged  that  a  current  entering  at  A  will  at  a  given  instant 
produce  a  south  pole  at  A  and  a  north  pole  at  B.  At  the  instant  shown 
at  /,  the  motor  poles  A  and  B  are  magnetized  while  poles  C  and  D  are 
not,  because  it  is  a  property  of  a  two-phase  circuit  that  when  the  current 

190 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS  191 

in  one  of  the  phases  is  at  a  maximum  value,  the  current  in  the  other 
phase  is  at  a  zero  value.  Hence,  the  bar  iron  rotor  will  assume  the 
vertical  position  shown. 

At  another  later  instant,  represented  at  //,  the  currents  in  both  of 
the  phases  are  equal  and  in  the  same  direction;  the  motor  poles  will 
be  magnetized  as  shown  and  the  rotor  will  be  drawn  into  the  position 
indicated.  At  the  instant  illustrated  at  ///,  because  of  the  properties 
of  two-phase  currents,  there  is  no  current  in  the  phase  the  conductors  of 
which  are  wound  ou  poles  A  and  B,  but  the  current  in  the  phase  the 
conductors  of  which  magnetize  poles  C  and  Z>,  is  a  maximum.  Hence 
the  rotor  is  now  drawn  into  a  horizontal  position.  Similar  action  occurs 
during  successive  instants  and  the  rotor  will  be  caused  to  rotate  in  the 
same  direction  within  the  motor  frame  so  long  as  the  two-phase  current 
is  applied  to  the  motor  terminals.  Considering  it  in  one  way,  the  rotat- 
ing magnetic  field  rotates  within  the  motor  frame  and  drags  the  rotor 
around  with  it. 

The  magnetic  attraction  or  drag  exerted  on  the  rotor  in  a  simple  motor 
built  as  illustrated  would  be  pulsating  in  effect,  hence  the  torque  exerted 
by  such  a  motor  would  not  be  uniform. 

298.  Commercial  Induction  Motors  operate  because  of  the 
principles  outlined  in  Art.  299,  but  their  construction  is  consid- 
erably different  from  that  shown  in  Fig.  196.     In  commercial 
induction  motors  the  stator  or  primary  winding  is  distributed 
over  the  entire  inner  surface  of  that  portion  of  the  stator 
structure  which  is  of  laminated  iron  and  which  conducts  the 
magnetic  flux.     The  rotor  consists  of  a  laminated  iron  cylinder 
which  has  a  winding  of  insulated  wire  or  of  copper  rods  or 
bars  embedded  in  slots  uniformly  spaced  around  the  periphery 
of  the  core.     Where  bars  or  rods  are  used  they  are  short-cir- 
cuited at  both  ends  by  heavy  copper  conductors  forming  a 
completely  short-circuited  rotor. 

299.  In  the   Commercial  Induction  Motor  the  Magnetic 
Field  of  the  Rotor  Which  Reacts  on  the  Magnetic  Field  of 
the  Stator  is  Produced  by  Currents  in  the  Rotor  Conductors. 

—These  currents  are  generated  by  the  rotor  conductors  being 
cut  by  the  lines  of  force  of  the  rotating  field  which  was  de- 
scribed in  a  preceding  paragraph.  Consider  a  polyphase  in- 
duction motor  with  its  rotor  at  rest.  Now  connect  a  source 
of  the  proper  polyphase  current  to  the  motor  terminals  thereby 
energizing  the  stator  winding.  A  rotating  magnetic  field  will 


192 


ELECTRICAL  MACHINERY 


[ART.  300 


Squirrel 

Cage 

IMMhw 


be  produced  by  the  stator  winding.  As  this  magnetic  field 
swings  around  within  the  stator  structure  it  will  cut  the  copper 
bars  imbedded  in  the  surface  of  the  rotor.  Currents  will 
thereby  be  induced  in  the  bars  and  these  currents  will  gen- 
erate magnetic  fields  around  and  within  the  rotor.  Due  to 
the  interaction  between  the  rotor  and  stator  magnetic  fields, 
rotation  of  the  rotor  will  be  produced. 

It  is,  therefore,  evident  that  the  turning  speed  (revolutions 
per  minute)  of  the  rotor  can  never  be  quite  equal  to  that  of 

the  rotating  magnetic  field  as 
there  must  always  be  a  suffi- 
cient difference  in  speed  or 
"slip"  (Art.  317)  that  the 
rotor  conductors  will  be  cut 
by  the  lines  of  force  of  the 
rotating  field.  Obviously,  if 
the  rotor  speed  were  the  same 
as  that  of  the  revolving  field, 
no  lines  of  force  could  be  cut 
by  rotor  conductors  and  there 
would  not  be  sufficient  mag- 
netic interaction  between  the 
stator  and  rotor  fields  to  pro- 
duce rotation  of  the  rotor  and 
pull  a  load.  The  intensity  of 
the  current  induced  in  the 


**-5lioleRail  ^Ball  Thrust  Bearing 

FIG.  197. — Sectional  elevation  of 
a  Westinghouse,  vertical,  induction 
cement-mill,  motor  (220  volts, 
three-phase,  60  cycles). 


rotor,  and,  therefore,  the 
torque,  is  determined  by  the 
amount  of  "slip,"  between  the 
rotor  and  the  rotating  mag- 
netic field.  The  greater  the  torque  required,  the  greater  will 
be  the  slip. 

300.  Special  Alternating-current  Motors  of  various  types 
are  manufactured.  Examples  of  these  are  the  types  adapted 
for  paper  mill,  textile  mill,  cement  mill,  (Figs.  197  and  198) 
mining  and  other  purposes.  The  vertical  cement-mill  motor 
illustrated  is  manufactured  in  capacities  of  from  75  to  200 
h.p.  and  operates  at  940  r.p.m. 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS 


193 


Fftff 


301.  General  Characteristics  of  Polyphase  Squirrel-cage 
Induction  Motors. — Their  speed  is  practically  constant  at  all 
loads.     Hence  they  are  used  for  constant-speed  service  where 
starting  and  reversing  are  infrequent.     The  starting  torque  is 
relatively  small  and  a  large  starting  current,  2  to  6  times  full- 
load  current,  depending  on  the  design  of  the  motor,  is  drawn 
from  the  line  if  the  motor  must  start  full-load  torque.     Simple 
and  rugged  construction  is  a  feature  of  these  motors,  the  bear- 
ings being  the  only  parts  subject  to  wear.     Since  there  are  no 
sliding  electrical  contacts  there 

can  be  no  sparking  and  the 
motors  are,  therefore,  particu- 
larly suitable  for  operation  in 
places  where  there  are  inflam- 
mable gases  or  dust. 

If  the  resistance  of  the  rotor 
be  increased  the  motor  can  be 
built,  in  the  smaller  capacities, 
for  high  starting  torque,  rapid 
acceleration,  and  frequent  star- 
ting. Motors  built  thus  can  be 
profitably  used  for  operating 
punches,  shears  and  the  like, 
where  simplicity  of  control  is 
desirable,  as  with  them  a  large 
drop  in  speed  produces  but  a 
slight  increase  in  torque,  per- 
mitting the  stored  energy  in 

the  flywheel  to  be  delivered  to  the  machine  when  a  heavy  load 
occurs.  In  this  respect  such  an  induction  motor  resembles  a 
compound- wound  direct-current  motor.  If  the  torque  imposed 
on  any  induction  motor  reaches  2  to  4  times  full-load  torque 
the  motor  will  stop  or  "  pull-out."  See  Art.  313.  Since  the 
output  and  torque  of  an  induction  motor  varies  as  the  square 
of  the  applied  voltage  it  is  desirable  to  maintain  the  voltage 
at  normal  value. 

302.  The  Electrical  Behavior  of  the  Polyphase  Induction 
Motor  on  the  input  side  is  the  exact  electro-magnetic  equivalent 

13 


FIG.  198.  Photographic  repro- 
duction of  Westinghouse  vertical 
cement-mill  motor. 


194  ELECTRICAL  MACHINERY  [ART.  303 

of  a  stationary  transformer  with  a  large  amount  of  magnetic 
leakage  between  the  primary  and  the  secondary  coils.  On  its 
output  side  the  polyphase  induction  motor  is  the  exact  electro- 
mechanical equivalent  of  the  continuous-current  shunt-wound 
motor  with  a  large  value  of  "armature  reaction."  The  con- 
necting link  between  the  output  and  the  input  sides  is  the 
magnetic  field,  which  may  be  considered  as  two  or  more  super- 
posed magnetic  fields  stationary  with  reference  to  each  primary 
coil  but  alternating  in  value  at  the  supply  frequency,  or  it 
may  with  equal  accuracy  and  convenience  be  considered  as 
a  single  resultant  field,  of  practically  constant  strength,  but 
revolving  in  space  with  reference  to  the  primary  circuit. 

303.  Factors  Affecting  the  Performance  of  the  Induction 
Motor.* — The  greater  the  maximum  output  or  breaking-down 
point  in  a  given  size  of  motor,  the  poorer  its  efficiency,  power- 
factor,  etc.,  at  normal  load.  To  get  excellent  all-round  results  it 
is  desirable  to  choose  a  reasonable  value  for  the  maximum  output 
of  the  motor.  The  characteristics  interesting  to  a  purchaser  are : 

1.  Efficiency,  that  is,  the  ratio  of  the  energy  given  out  by 
the  motor  to  the  energy  put  in. 

2.  Maximum  output,  that  is,  the  greatest  horsepower  that 
the  motor  will  carry  without  unduly  slowing  down,  or  perhaps 
stopping  altogether. 

3.  Current  taken  at  the  instant  of  starting,  sometimes  called 
impedance  current.     When  the  switch  is  closed,  impressing 
e.m.f.  on  an  induction  motor,  there  is  (unless  provisions  are 
adopted  to  prevent  it)  a  rush  of  current  which  may  cause  dis- 
turbances of  voltage  on  the  line  to  which  the  motor  is  con- 
nected, thus  giving  occasion  for  complaints,  especially  if  the 
circuit  is  supplying  lights.     If  no  lights  are  on  the  circuit,  the 
disturbance  may  extend  to  other  motors  on  the  circuit,  causing 
trouble  with  them. 

4.  Current  Taken  when  Running  without  Load. — With  an  in- 
duction motor,  the  field  is  produced  by  a  current  drawn  from 
the  line  through  the  same  wires  that  supply  the  energy.     This 
current,  called  the  magnetizing  or  wattless  current,  lags  behind 
the  electromotive  force,  and  pulls  down  the  voltage  of  the 

*  From  Raymond's  MOTOR  TROUBLES. 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS  195 

circuit,  from  which  it  flows,  to  a  much  greater  extent  than  does 
an  energy  current,  so  that  it  is  desirable  to  have  as  small  a 
magnetizing  current  as  possible.  For  a  well-designed  station- 
ary motor  for  ordinary  purposes,  this  no-load  current  should 
be  not  over  30  per  cent,  of  the  total  full-load  current. 

5.  The  power-factor,  see  the  author's  PRACTICAL  ELECTRICITY, 
which  is  the  ratio  of  the  component  of  the  current,  represent- 
ing energy,  to  the  total  current  flowing  into  the  motor  (the 
total  current  being  the  resultant  sum  of  the  energy  current  and 
the  magnetizing  current) .  Thus,  in  a  motor  taking  1 ,500  watts 
per  phase  (about  2  h.p.),  at  100  volts  per  phase  with  a  mag- 
netizing current  of  6  amp.  per  phase,  the  total  current  per  phase 
flowing  into  the  motor  is  found  approximately  as  follows: 


(56)          Total  current  =  ^G2  +  =  16  amp. 


EXAMPLE.  —  Let  1,  2,  Fig.  199,  equal  the  electromotive  force  applied 
to  the  phase;  let  1,  5  represent,  in  direction  and  in  length,  the  energy 
component  of  the  current,  and  1,  4 
the  magnetizing  current;   then  line 
1,  3  represents  the  total  current,  or    Magnetizim 
resultant  of  the  currents  1,  4  and  1,      Current-^ 
5.     This  combination  of  currents  is  i  ^  £nergy  Component- 

characteristic  of  alternating  circuits.       ^  m_Phase  relation  of  cur. 
That  is,  currents  combine  directly   rents    jn    an    alternating-current 
only  when  in  phase.      When  not  in    motor. 
phase,   their  resultant    is   obtained 

with  the  parallelogram  of  forces,  as  shown  with  1,  4  and  1,  5  in  Fig. 
199.  The  power-factor,  therefore,  is  the  ratio  of  the  line;  1,  5  to  1,  3,  or 
the  ratio  of  the  energy  component  of  the  current  to  the  total  current  and 
this  can  be  roughly  estimated  as  shown. 

304.  Loss  of  Efficiency  of  an  Induction  Motor  at  Reduced 
Speeds.*  —  When  an  induction  motor  is  operated  at  reduced 
speeds  by  increasing  the-  slip,  as  by  increasing  the  secondary 
resistance  or  decreasing  the  primary  voltage,  the  efficiency  is 
lowered  by  an  amount  nearly  proportional  to  the  speed  reduc- 
tion, as  expressed  by  the  formula: 


(57)  E2  =  E,--  (efficiency) 

*  B.  G.  Lamme,  National  Electric  Light  Association  Convention,  Niagara  Falls,  June, 

1897, 


196 


ELECTRICAL  MACHINERY 


[ART.  305 


in  which  EI  =  the  efficiency  of  the  motor  when  running  at  a 
given  torque  and  a  slip  Si.  E2  =  the  efficiency  when  the 
motor  is  developing  the  same  torque  but  with  a  different  slip 
$2,  efficiencies  and  slips  being  expressed  in  per  cent. 

EXAMPLE. — Suppose  a  given  60-cycle,  8-pole  motor  with  a  synchronous 
speed  of  900  r.p.m.  has  a  normal  full-load  speed  of  855  r.p.m.  (slip  5  per 
cent.)  and  an  efficiency  of  90  per  cent.,  the  efficiency  at  810  r.p.m'.  (slip 
10  per  cent.)  with  full-load  torque  will  be: 


/100  - 
E2  =  90 


MOO 


LO) 
-5  / 


90 

90  X  —   =  85.2  per  cent. 
95 


305.  Effect  of  Changes  in  Voltage  and  Frequency  on  Induc- 
tion-motor Operation.* — Some  variations  from  normal  voltage 
and  frequency  are  generally  permissible  with  any  induction 
motor,  but  such  variations  are  always  accompanied  by  changes 
from  normal  performance.  With  either  the  voltage  or  the 
frequency  differing  from  normal  the  following  performance 
changes  must  be  expected: 


Conditions 

Power-factor 

Torque 

Slip 

Voltage  high 

Decreased 

Increased 

Decreased 

Voltage  low 

Increased 

Decreased 

Increased 

Frequency  high 

Increased 

Decreased 

Per  cent,  slip  un- 

changed. 

Frequency  low 

Decreased 

Increased 

Per  cent,  slip  un- 

changed. 

Usually  a  variation  of  either  voltage  or  frequency  not  ex- 
ceeding 10  per  cent,  is  permissible,  and  within  this  limit  the 
efficiency  remains  approximately  unchanged.  The  voltage  and 
frequency  should  not  be  varied  simultaneously  in  opposite 
directions,  that  is,  one  decreased  and  the  other  increased.  If 
an  induction  motor  must  operate  on  frequency  other  than  stand- 
ard, the  performance  will  be  better  if  the  voltage  is  changed 
in  proportion  to  the  square  root  of  the  frequency.  Thus  a  400- 
volt,  60-cycle  motor  operating  on  66%  cycles  will  have  very 

*  Westinghouse  Elec.  &  Manfg.  Co.  INSTRUCTION  BOOK. 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS  197 

nearly  its  normal  operating  characteristics  if  the  voltage  is 
raised  to: 


400  X  \/          =  442  volts.     Decreasing  the  voltage  much  be- 


low  normal  is  seldom  permissible  on  account  of  resulting  in- 
creased temperature  rises. 

306.  Tables  of  Performance  Data  for  Polyphase  Induction 
Motors  of  standard  capacities  ranging  from  J£  to  200  h.p.,  are 
given  in  the  author's  AMERICAN  ELECTRICIANS'  HANDBOOK. 
In  these  tables  are  shown  the  synchronous  speeds,  per  cent, 
slips,  approximate  full-load  speeds,  full-load  currents,  starting 
currents,  starting  torques,  efficiencies  and  power-factors  for 
the  motors  of  the  various  ratings  and  standard  voltages. 

307.  Performance     Guarantees     on     Alternating-current 
Motors  are  now  usually  made  on  the  so-called  "normal"  basis. 
However,  it  is  probable  that  all  motors  made  in  this  country 
will  shortly  be  rated  on  the  continuous  (Art.  47a)  basis.     The 
normal  rating  usually  given  these  motors  specifies  that  they 
will  operate  continuously  at  their  horse-power  rated  (name- 
plate)  outputs  with  a  temperature  rise  not  to  exceed  40  deg. 
C.  and  furthermore  that  they  will  operate  at  a  25  per  cent, 
overload  for  two  hours  with  a  temperature  rise  not  to  exceed 
55  deg.  C.     All  of  the  above  temperature  rises  are  based  on 
a  "room"  reference  or  ambient  (Art.  476)  temperature  of  25 
deg.  C. 

308.  Characteristics  of  Polyphase  Induction  Motors  Having 
Wound  Rotors  and  Internal  Starting  Resistance. — Motors  of 
this  type  of  the  ordinary  design  give  about  1^  times  full-load 
torque  with  approximately  1^  'times  full-load  current,  making 
them  suitable  for  use  on  lighting  circuits  and  for  other  applica- 
tions where  a  minimum  starting  current  is  desirable.     In  gen- 
eral, motors  of  this  type  are  not  built  in  capacities  exceeding 
200  h.p.  because  of  the  mechanical  difficulties  encountered  in 
arranging  the  internal  resistance. 

309.  Compared  with  the  Squirrel-cage  Motor,  One  with  a 
Wound  Rotor  and  internal  resistance  will  develop  a  greater 
starting  torque  per  ampere,  but  it  should  not  be  used  for  ap- 
plications involving  great  inertia  or  excessive  static  friction. 


108 


ELECTRICAL  MACHINERY 


[ART.  310 


If  used  for  such  applications,  full  starting  current  may  be  re- 
quired for  a  considerable  period  before  the  apparatus  attains 
full  speed.  Since  the  capacity  of  the  internal  resistance  is 
small,  excessive  temperatures  may  result  and  cause  trouble. 

310.  Characteristics  of  Polyphase  Slip-ring  or  Wound-rotor 
Induction  Motors  Having  External  Starting  Resistance. — 
These  motors  have  insulated  wire  or  bar  windings  on  the  rotor 
and  are  provided  with  collector  rings  whereby  an  external 
resistance  can  be  connected  in  the  rotor  circuit.  The  speed 
of  the  motor  can  be  varied  by  varying  the  amount  of  external 
resistance  in  the  rotor  circuit.  These  motors  may  be  used  in 
moderate  and  large  capacities  for  nearly  all  variable-speed  ap- 


5   10  15  20  15  30  35  40  45  50 
Horsepower  Output. 

FIG.  200. — Typical   performance 


Horsepower  Output. 


FIG.  201. — Performance    graphs 


raphs  of  a  20-h.p.,  three-phase  in-    of  the  motor  shown  in  Fig.   202, 


luction  motor. 


when  running  single-phase. 


plications.  They  are  also  used  for  constant-speed  applications 
where  the  starting  current  must  be  low.  The  motors  operate 
with  characteristics'  similar  to  those  of  direct-current  motors 
having  resistance  in  the  armature  circuit.  When  the  external 
resistance  is  short-circuited,  the  motors  really  become  squirrel- 
cage  machines  and  operate  with  the  characteristics  of  such 
machines. 

311.  Characteristic  Graphs  of  the  Induction  Motor. — Those 
of  Fig.  200  are  fairly  typical  of  the  average  commercial  in- 
duction motor.  It  will  be  noted  that  the  normal  rating  of  the 
motor  is  taken  at  such  a  point  that  both  the  power-factor 
and  the  efficiency  are  the  highest  possible.  The  motor  could 
be  so  designed  that  either  the  power-factor  or  the  efficiency, 


SEC.  8] 


INDUCTION  AND  REPULSION  MOTORS 


199 


but  not  both,  could  be  higher  than  shown  at  normal  load, 
but  the  design  of  an  induction  motor  is  a  compromise  result- 
ing in  the  best  efficiency  and  power-factor  obtainable  with  suit- 
able overload  and  starting  characteristics.  Fig.  201  shows  the 
curves  of  the  same  motor  running  single-phase. 

312.  The  Torque  Graphs  of  an  Induction  Motor  with  a 
Wound  Rotor,  from  rest  to  synchronism,  running  both  three- 
phase  and  single-phase  with  resistance  and  without  resistance, 
are  shown  in  Fig.  202.  Graph  A  shows  the  torque  from  rest 
to  synchronism  without  resistance  in  the  rotor  circuit.  If 
resistance  is  inserted,  graph  B  is  obtained  and  the  starting 
torque  is  440  Ib.  against  170  Ib.  without  resistance.  It  should 


1" 


0    10    20  30  40    50  60  70   80  90   100 
Percent  Synchronism. 


10    ZO  30  40   50   60    70    80   90   100 
Percent  Synchronism. 


FIG.  202.—  Torque  graphs  of  a  30-        FIG.  203.  —  Torque  graph  of  a  1- 
h.p.  induction  motor.  h.p.,   three-phase   induction   motor, 

running  single-phase. 

be  noted  that  the  "  pounds  torque"  values  given  in  the  graphs 
of  Figs.  202  to  205  represent  the  pounds  torque  at  a  1-ft. 
radius  and  are,  therefore,  equivalent  to  "  pound-foot  torque. 
See  Art.  238.  Graph  C  indicates  the  torque  where  too  much 
resistance  is  used  in  the  rotor.  Graph  E  illustrates  the 
torque  single-phase,  which  is  zero  at  starting.  An  induction 
motor  starts  as  shows  on  graph  B  until  it  reaches  the  point 
F,  when  the  resistance  is  cut  out  and  the  motor  adjusts 
itself  to  its  operating  position  at  G.  Thus,  if  the  torque  re- 
quired of  the  motor  for  which  the  graph  is  shown,  is  greater 
than  440  Ib.,  shown  at  H,  the  motor  will  break  down  and 
come  to  rest.  With  the  resistance  in  the  rotor,  a  starting 
torque  of  440  Ib.  is  available,  but  this  load  cannot  be  brought 


200 


ELECTRICAL  MACHINERY 


[ART.  313 


up  to  normal  speed.  The  motor  can  only  bring  the  torque 
represented  by  the  point  F,  in  other  words  290  lb.,  up  to 
normal  speed. 

In  Fig.  203  it  will  be  noted  that  the  torque  of  a  three- 
phase  motor  running  single-phase  at  starting  is  zero,  rising  to 
a  maximum  and  reaching  zero  at  synchronism.  This  means 
that  an  induction  motor  never  runs  at  synchronous  speed. 
The  three-phase  motor,  Fig.  204,  starts  with  a  reasonable 
torque,  reaches  its  maximum  output  and  goes  to  zero  again 
at  synchronism. 

Figs.  204  and  205  show  the  torque  curves  of  squirrel-cage 
motors  without  resistance  in  the  rotor  circuit.  With  resistance 
inserted  in  the  armature,  the  torque  is  greater  at  starting  and 


|400 
£300 
•£  200 

j§  10° 
0 

s, 

^ 

^ 

s* 

** 

^ 

\ 

^ 

\ 

! 

J    20  30  40    50   60  70  80  90  100- 

Percent  Synchronism. 


10   20  30  40  50  60  70  80  90  100 

Percent  Synchronism. 

FIG.  204. — Torque  graph  of  a  20-  FIG.  205. — Torque  graph  of  a  1- 
h.p.,  three-phase  induction  motor,  h.p.,  three-phase  induction  motor. 

less  later.  This  is  the  reason  that  it  is  advantageous  to  in- 
troduce resistance  at  starting  and  cut  it  out  as  synchronism 
is  approached. 

313.  The  Pull-out  Torque  of  an  Induction  Motor.— All  in- 
duction motors  will  "pull  out"  at  some  certain  torque  if  they 
are  overloaded.     The  " pull-out"  limit — the  maximum  torque 
that  can  be  developed — is  that  point  at  which  further  increase 
in  torque  will  cause  the  motor  speed  to  decrease  rapidly  and 
then  to  stop.     This  point  is  usually  at  between  2  and  4  times 
the  full-load  rated  torque,  depending  on  the  design  and  the 
capacity  of  the  motor.     See  the  typical  induction-motor  curve, 
Fig.  200. 

314.  Starting  Torque  and  Starting  Current  of  Alternating- 
current  Motors.  * — In  what  follows  the  starting  torque  is  ex- 

•  F.  D.  Newbury,  N.  E.  L.  A.  Convention  Paper,  1911. 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS  201 

pressed  in  terms  of  the  full-load  torque,  and  the  starting  cur- 
rent in  terms  of  the  full-load  current.  The  smaller  values  given 
for  synchronous  motors  cover  the  requirements  of  motor-gen- 
erator sets  and  air  compressors  and  pumps  when  the  apparatus 
can  be  started  without  load.  The  larger  values  refer  to 
motors  for  driving  pumps  and  fans,  which  must  be  started 
under  practically  full-load  conditions.  The  wide  variation  in 
the  starting  current  comes  from  differences  in  construction  of 
the  motor  or  differences  in  the  proportions  of  the  motor,  since, 
by  increasing  the  size  and  cost  of  synchronous  motors,  the 
starting  performances  can  be  materially  improved. 

SINGLE-PHASE  INDUCTION  MOTORS,  WITH  CLUTCH,  SPLIT-PHASE 
STARTER.  —  Starting  torque,  1  to  1  J4,  starting  current,  4J^  to  6. 

SINGLE-PHASE  INDUCTION  MOTORS,  WITHOUT  CLUTCH,  SPLIT- 
PHASE  STARTER.  —  Starting  torque,  2;  starting  current,  3^  to 


POLYPHASE  INDUCTION  MOTORS,  CAGE-WOUND  TYPE,  AUTO- 
TRANSFORMER  STARTER.  —  Starting  torque,  2;  starting  current, 
7  to  8. 

POLYPHASE  INDUCTION  MOTORS,  WOUND-ROTOR  TYPE,  STEP- 
BY-STEP  RESISTANCE  STARTER.  —  Starting  torque,  1;  starting 
current,  \Y±.  Starting  torque,  2;  starting  current,  2j^. 

SYNCHRONOUS  MOTORS,  AUTO-TRANSFORMER  STARTER.  — 
Starting  torque,  0.3  to  0.5;  starting  current,  1J^  to  2%. 
Starting  torque,  0.7  to  1;  starting  current,  4  to  8. 

ROTARY  CONVERTERS,  AUTO-TRANSFORMER  STARTER.  -  Start- 

ing torque,  0.2;  starting  current,  1J£.     Starting  torque,  suffi- 
cient to  start  itself. 

315.  The  Relations  Between  Speed,  Frequency  and  Number 
of  Poles  of  an  Alternating-current  Motor  of  the  Synchronous 
or  Induction  Type  follows  from  the  equation  for  alternating- 
current  generators  given  in  Art.  253.  For  synchronous  motors, 
these  equations  of  Art.  253  are  correct,  but  for  induction  motors 
which  do  not,  when  loaded,  operate  at  synchronous  speeds, 
the  following  formulas  should  be  used: 

120  X  /  X  (1.00  -  *) 
(58)  r.p.m.  =  -  —±-  (speed) 


202  ELECTRICAL  MACHINERY  [ART.  316 

/cn\  f  P  *  r.p.m.  ,f  . 

=  120  X  (1.00  -  s)  (frequency) 


(60)  ..1.00- 

,A1.  120  X/  X  (1.00  -  s) 

(61)  P  =  ~  -  -  (number  of  poles) 

Wherein,  /  =  frequency  in  cycles  per  second,  r.p.m.  =  revo- 
lutions per  minute  of  rotor,  p  =  number  of  poles,  s  =  the 
slip,  expressed  decimally.  The  method  of  determining  slip  is 
described  in  Art.  317.  In  the  author's  AMERICAN  ELECTRI- 
CIANS' HANDBOOK  will  be  found  a  table  showing  the  per  cent. 
slip  at  full-load  for  25-  and  60-cycle  motors  of  the  commonly 
used  capacities  and  speeds. 

EXAMPLE.  —  What  will  be  the  speed  of  a  6-pole,  60-cycle,  induction 
motor  at  full-load,  if  its  full-load  slip  is  known  to  be  7  per  cent!? 
SOLUTION.  —  Substitute  in  the  equation  (1):  r.p.m.  =  (120  X  /  X  [l.OO 
-  s])  4-  6  =  (120  X  60  X  [1.00  -  0.07])  •*-  6  =  6,696  -=-  6  =  1,116  r.p.m. 

316.  Speed  Regulation  of  Induction  Motors.     Slip.  —  The 
speed  regulation  is  the  percentage  drop  in  speed  between  no- 
load  and  full-load  based  on  the  maximum  speed;  it  is  usually 
called  the  "slip."     The  "slip"  at  full-load  is  usually  about  5 
to  7  per  cent.     At  other  loads  it  is  approximately  proportional 
to  the  load,  therefore,  at  twice  full-load  the  drop  in  speed  will 
be  approximately  10  to  15  per  cent.     Refer  to  the  author's 
AMERICAN  ELECTRICIANS'  HANDBOOK  for  a  table  showing  the 
slips  to  be  expected  from  commercial  induction  motors. 

317.  The  Slip  of  an  Induction  Motor  is  the  ratio  of  the 
difference  between  the  rotating  magnetic-field  speed  (revolu- 
tions per  minute  or  angular  velocity)  and  the  rotor  speed  to 
the  rotating  magnetic-field  speed.     The  speed  of  the  rotating 
magnetic  field  is  equivalent  to  the  synchronous  speed  (Art. 
253)  of  the  machine  (see  table  of  synchronous  speeds  in  the 
AMERICAN  ELECTRICIANS'  HANDBOOK)  which  is  determined  by 
the  frequency  of  the  current  and  the  number  of  poles  of  the 
machine.     Then  : 

,.  _.  synchronous  speed  —  actual  speed 

(b2)  blip  =  -  ;  -  1  - 

synchronous  speed 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS  203 

When  there  is  no  load  on  a  motor  the  slip  is  very  small, 
that  is,  the  rotor  speed  is  practically  equal  to  the  synchronous 
speed. 

EXAMPLE. — What  is  the  slip  at  full-load  of  a  4-pole,  60-eycle  induction 
motor  which  has  a  full-load  speed  of  1,700  r.p.m.  SOLUTION. — From 
formula  23  the  speed  of  the  rotating  field  or  the  synchronous  speed  of 
a  4-pole,  60-cycle  motor  is  1,800  r.p.m.  Then  substituting  in  the  above 
formula: 

sync,  speed  —  act.  speed       1,800  -  1,700         100 

Slip  =  -  -  =  J  -  =  -     -  =  5.5  per  cent, 

sync,  speed  1,800  1,800 

Therefore  the  slip  is  5.5  per  cent.  The  voltage  of  the  motor  or  whether 
it  is  single-phase,  two-phase,  or  three-phase  are  not  factors  in  the 
problem. 

318.  The  Induction  Motor  Inherently  a  Constant-speed 
Motor.  The  Regenerative  Feature.* — A  characteristic  of  the 
induction  motor  is  that  it  tends  to  rotate  at  a  definite  syn- 
chronous speed  irrespective  of  whether  the  motor  is  driving  or 
being  driven,  providing  there  is  no  starting  resistance  in  the 
rotor  circuit.  For  illustration,  when  a  load  is  being  lowered 
and  the  motor  is  connected  to  a  source  of  energy,  it  acts  as  an 
alternating-current  generator,  the  descending  load  furnishing 
the  driving  power.  The  motor  delivers  energy  to  the  line. 
When  load  is  being  raised  the  motor  absorbs  energy  from  the 
line.  This  returning  of  energy  to  the  line  by  a  motor  is  termed 
regeneration.  Consider  an  installation  where  cars  loaded  with 
ore  are  lowered  down  a  slope  on  a  railroad  and  the  empty  cars 
are  hoisted  back.  The  motor  delivers  about  as  much  power 
to  the  line  when  lowering  as  it  consumes  when  hoisting,  with 
the  result  that  practically  no  energy  is  consumed  in  operating 
the  system.  The  proof  of  this  is  that  the  watt-hour  meter 
for  such  an  installation  runs  backward  about  as  much  as  it 
runs  forward. 

EXAMPLE. — Another  interesting  example  is  a  balanced  passenger  hoist 
wherein  the  passenger  cars  run  over  varying  grades  and  sometimes  one 
is  loaded,  at  other  times  the  other  is  loaded.  The  cars,  when  equipped 
with  induction  motors  connected  to  a  source  of  energy,  run  at  a  practi- 
cally uniform  speed  without  the  use  of  brakes,  whether  the  load  overhauls 
the  motor  or  not.  This  characteristic  will  not  obtain  if  starting  resist- 

*  PRACTICAL  ENGINEER. 


204 


ELECTRICAL  MACHINERY 


[ART.  319 


ance  is  included  in  the  rotor  circuit,  for  then  the  motor  will  slow  down 
when  it  is  delivering  power  to  the  cars  and  will  operate  at  an  overspeed 
if  the  cars  are  delivering  power  to  the  motor. 

319.  To  Compute  Either  the  Horse-power  Output,  Current, 
Voltage,  Power-factor  or  Efficiency  of  any  Three-phase,  Al- 
ternating-current Motor,  the  other  quantities  being  known, 
one  of  the  following  formulas  may  be  used: 

E*  X  /»  X  pj.  X  E 

(horse-power) 

(volts) 
(amperes) 
(power-factor) 
(efficiency) 


horsepower.  E3 
=  voltage  between  any  two  of 
the  three  wires  of  the  balanced 
three-phase  system.  J3  =  cur- 
rent, in  amperes,  in  each  of  the 
three  wires  of  the  three-phase 
system,  p.f.  =  power-factor  of 
the  motor,  expressed  decimally. 
E  =  efficiency  of  the  motor,  ex- 
pressed decimally. 

EXAMPLE. — What  will  be  the  horse- 
power  output  of   a  220-volt,  three- 


^uo; 

(64) 
(65) 
(66) 

.(67) 
Wherein, 

n.y.o 

E3 

/3 
P.f. 

E 

h.p.0  = 

430.7 
h.p.o  X  430.7 

/3  X  p.f.  X  E     . 
h.p.0  X  430.7 

E3  X  p.f.  X  E 
h.p.o  X  430.7 

73  X  E3  X  E 
h.p.0  X  430.7 

p.f.  X  h  X  Es 
power  output  of  the  mote 

Voltmeters' 
Read  2?0-Vo/ts 

FIG.  206. — Example  in  deter- 
mining horsepower  output  of  a 
three-phase  motor. 


phase  induction  motor,  Fig.  206,  if  its'  efficiency  at  full-load  is  known 
to  be  87  per  cent.,  its  full-load  power-factor  85  per  cent.,  and  its  full- 
load  current  88  amp.?  SOLUTION. — Substitute  in  equation  (63): 
h.p.0  =  (E*  X  /3  X  p.f.  X  E)  -*•  430.7  =  (220  X  88  X  0.85  X  0.87)  -^ 
430.7  =  14,316.72  -*•  430.7  =  33.2  h.p. 

EXAMPLE. — The  three-phase  induction  motor  shown  in  Fig.  207  is 
known  to  be  delivering  50  h.p.  at  the  pulley,  P.  The  impressed  e.m.f. 
is  440  volts.  The  power-factor  at  this  load  is  known  to  be  86  per  cent. 
and  the  efficiency  90  per  cent.  What  current  is  being  taken  by  the 
motor  under  these  conditions?  SOLUTION. — Substitute  in  equation  (65) : 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS 


205 


73  =  (A.p.o  X  430.7)  -h  (E3  X  p.f.  X  E)  =  (50  X  430.7)  -H  (440  X  0.86 
X  0.90)  =  21,535  H-  340.56  =  63.2  amp. 

EXAMPLE. — The  full-load  output  of  the  220-volt,  three-phase  induc- 
tion motor  shown  in  Fig.  208  was  found  by  the  Prony  brake  test  to  be 
35  h.p.  Under  these  conditions  the  efficiency  was  88  per  cent.,  and  the 
motor  took  88  amp.  in  each  of  the  three-phase  wires.  What  was  its 


Driven  Machine* 


,  -  -Three-Phase  Motor 
Power  Fac  tor"  86  % 
Efficiency  =  90% 


"^Delivering  50  h.p.  Here 


Nicjy 
FIG.  207. — Example  in  computing  current  taken  by  three-phase  motor. 


power-factor  under  these  conditions?  SOLUTION. — Substitute  in  equation 
(66):  p.f.  =  (h.p.0  X  430.7)  -=-  (/,  X  E3  X  E)  =  (35  X  430.7)  -H  (88 
X  220  X  0.88)  =  15,074.5  -r-  17,036.8  =  0.884.  Hence,  the  power- 
factor  of  this  motor  at  35  h.p.  is  88.4  per  cent. 


.'•Three  -Phase  Induction 
Motor.  Delivering  35  h.p.  at  ah 
Efficiency  of  88  per  cent 


FIG.  208. — Example  in  determining  by  test  the  power  factor  of  a  three- 
three-phase  induction  motor. 


320.  To  Compute  Either  the  Horse-power  Output,  Current, 
Voltage,  Power-factor  or  Efficiency  of  any  Two-phase  Alter- 
nating-current Motor,  the  other  quantities  being  known,  the 
following  formula,  or  one  derived  from  it,  may  be  used: 

E2  X  /2  X  p.f.  X  E 

(68)  h.p.0  =  -  r.  (horse-power) 

o/o 

Wherein,  all  of  the  symbols  have  the  same  meanings  as  in  Art. 
319,  except  that  Ez  =  (for  a  four- wire,  two-phase  system), 
voltage  between  phase  wires,  and  (for  a  three-wire,  two-phase 
system)  =  0.707  X  voltage  between  each  phase  wire  and  the 
neutral. 


206 


ELECTRICAL  MACHINERY 


[ART.  321 


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SEC.  8]          INDUCTION  AND  REPULSION  MOTORS  207 

322.  Single-phase  Motors  of  the  types  ordinarily  manufac- 
tured and  used  in  power  practice  may  be  arbitrarily  classified 
into  four  different  groups  as  shown  in  Table  321.     No  at- 
tempt is  made  herein  to  consider  single-phase  railway  motors. 
Other  classifications  can  be  made  which  may,  for  some  pur- 
poses, be  preferable  to  the  one  shown.     No  generally  adopted 
standard  of  classification  is  available  at  present.     However, 
it  is  understood  that  the  Electric  Power  Club  and  other 
electrical  societies  cooperating  in  an  endeavor  to  compile  a 
classification  of  single-phase  power  motors  which  will,  doubt- 
less, ultimately  be  adopted  as  a  standard   classification   in 
this  country. 

323.  A    Single-phase  Induction  Motor  pure   and   simple 
(classification   la  in  321)  develops  no  starting  torque  when 
its  rotor  is  not  revolving.     However,  if  the  revolution  of  the 
motor  is  started  by  some  means  or  other,  there  is  a  certain 
interaction  of  magnetic  fields  whereby  there  is  exerted  on  the 
shaft  a  continuous  turning  effort.     While  a  single-phase  in- 
duction motor  may  be  started  by  hand  by  giving  the  rotor  a 
twist,  obviously,  such  a  method  of  starting  a  motor  is  not 
feasible  for  commercial  machines;  hence  others,  whereby  the 
motor  can  be  started  automatically,  are  adopted  in  practice. 
All  of  these  methods  of  automatic  starting  (except  that  using 
a  shading  coil,  Art.  330)  involve  electromagnetic  interactions 
of  some  sort  which  occur — with  the  so-called  straight  induc- 
tion motors — only  during  the  starting  period.     In  this  dis- 
cussion,   when    a    straight   single-phase  induction  motor  is 
referred  to,  the  term  is  used  to  designate  one  with  a  squirrel- 
cage  rotor.     After  the  motor  is  started  it  then  operates  as 
a   single-phase   induction  motor,   pure   and   simple.     These 
methods  of  starting  may  be  classified  thus:   (1)  split-phase 
method;  (2)  shading  coil. 

324.  In  the  Split-phase  Method  of  Starting  a  Single-phase 
Induction  Motor,  the  motor  (Figs.  209,  210,  211  and  212)  is, 
in  practice,  provided  with  two  distinct  windings  called  the 
starting  and  the  running  windings  (A  and  W,  respectively, 
Fig.  211).     The  starting  winding  circuit  is  so  arranged  that  it  has 
considerably  more  inductance,  resistance  or  capacity — usually 


208 


ELECTRICAL  MACHINERY 


[ART.  325 


End., 
Bell 


resistance — than  had  the  running  winding.  Furthermore,  the 
starting  winding  is  displaced  in  the  stator  by  90  electrical 
degrees,  as  shown  in  Fig.  211,  from  the  running  winding.  Due 
to  the  excess  of  inductance,  resistance  or  capacity  the  current 

in  the  starting  winding  will  differ 
considerably  in  phase  from  the 
current  in  the  running  winding. 
Because  of  this  condition,  and  of 
the  relative  positions  of  the  two 
windings,  a  rotating  field  is  pro- 
duced in  the  motor  during  the 
starting  period,  somewhat  simi- 
lar to  the  rotating  field  produced 
in  a  two-phase  induction  motor. 
325.  In  Practice  the  Running 
Winding,  Figs.  211  and  212,  usu- 
ally consist  of  a  considerable 
number  of  turns  of  large  wire, 
well  distributed  over  the  stator. 


Leads' 


FIG.  209. — Typical  construc- 
tion of  a  split-phase-starting, 
single-phase  motor. 


Practically  all  of  the  split-phase  motors  on  the  market  use  a 
starting-winding  circuit  of  high  resistance.  Hence,  the  starting 
winding,  ordinarily  consists  of  fine  wire,  thus  giving  this  wind- 
ing a  high  resistance.  In  fan  motors  of  some  designs  an  in- 


rifU  and 
Starting  Coils 


•frame 


.Shaft 


^•Centrifugal  Switch 

FIG.  210. — A  disassembled  view  of  a  split-phase-starting  motor. 

ductance  coil  mounted  in  the  base  of  the  motor  is  connected  in 
series  with  the  starting  winding  to  provide  the  necessary  induc- 
tance. The  running  winding  remains  in  circuit  at  all  times 
when  the  motor  is  not  in  operation.  But  the  starting  winding 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS 


209 


remains  in  circuit  only  until  the  speed  of  the  rotor  appoaches 
synchronous  speed.  When  this  speed  is  attained  then  the  rotor 
winding  should  be  cut  out,  an  automatic  centrifugal  switch  (Fig. 
212  and  Fig.  211,  Si,  S2)  operates.  This  opens  the  starting  cir- 


Winc/ing';_ 


I- Elements 


I-Diagram 


FIG.  211. — Diagrammatic   representations   of   "straight"   single-phase 
induction  motor. 

cuit  and  then  the  motor  continues  to  operate  as  a  squirrel- 
cage  induction  motor,  solely  by  virtue  of  its  running  winding 
and  circuit  and  its  squirrel-cage  rotor.  In  small  single-phase 
induction  motors  of  certain  manufacture,  the  squirrel-cage 


Motor., 


Rotates  with  Rotor-, 


Running  Winding 


Automatic}   ' 
Centrifugal}  .(. 
Cut-out  ) 
Shaft— 


Starting 

Winding"**" 


^Connection  ^faffi"* 

U/OCff 


Circuit. 


. 

Stationary 


Starting  Winding 

Cut  out  of  Circuit 

Automatic    Switch 

FIG.  212. — Single-phase-motor  diagram. 

element  is  arranged  in  the  stator  and  the  running  and  starting 
windings  are  arranged  on  the  rotor. 

326.  The  Principle  of  the  Split-phase  Method  of  Starting 
is  illustrated  in  Fig.  213,  which  shows  an  explanatory  diagram 

14 


210 


ELECTRICAL  MACHINERY 


[ART.  327 


To  AC. 
Line 


and  not  a  commercial  motor.  In  starting  one  of  these  motors, 
first  the  main  switch,  M,  is  closed,  which  excites  the  running 
winding,  W.  However,  with  only  this  running  winding  ex- 
cited, the  motor  will  not,  of  itself,  start,  but  if  now  the  start- 
ing switch  S  is  closed,  which  energizes  the  starting  winding, 
A,  which  has  in  series  with  it  the  resistance,  R,  the  rotating- 
fi eld  effect  referred  to  above  is  thereby  produced  in  the  machine 

and  the  rotor  will  commence  to 
revolve.  When  the  motor 
attains  a  speed  which  approaches 
synchronous  speed,  the  switch 
S  may  then  be  opened  and  the 
motor  will  continue  to  operate  as 
an  induction  motor. 

327.  The  Split-phase  Method 
of  Starting  Induction  Motors  is 
Used  Only  for  Motors  of  Small 
Capacity. — Most  of  the  induction 
fan  motors  and    the  fractional 
horse-power  motors  employ  this 
method  of  starting.     However, 
the  repulsion  method  of  starting 
(Art.  336)  is  now  being  used  to 
some  extent  for  starting   frac- 
tional  horse-power   motors  be- 
cause of  the  more  desirable  star- 
ting    characteristics     which    it 
affords. 

328.  As    to  the    Starting 
Torque,    Starting  Current  and 

Speed  Regulation  of  Single -phase,  Phase -splitting-starting  In- 
duction Motors,  they  are  suitable  for  applications  for  which  the 
starting  torque  required  is  not  over  150  per  cent,  of  full-load 
torque.  The  starting  current  for  a  motor  designed  to  develop 
150  per  cent,  full-load  torque  is  approximately  550  per  cent, 
of  full-load  current.  The  maximum  torque  is  from  200  to  250 
per  cent,  of  the  full-load  torque.  The  speed  regulation 
from  no-load  to  full-load  is  good — better  than  with  the 


Squirrel- Cage  Potor» 

FIG.  213.— "Straight"  single- 
phase  motor  with  an  external 
starting  reactance. 


SEC.  8] 


INDUCTION  AND  REPULSION  MOTORS 


211 


polyphase  motor.  In  general,  however,  the  efficiency,  pow- 
er-factor and  maximum  torque  are  not  as  good  as  in  cor- 
responding polyphase  motors.  They  are  suited  only  for 
driving  machinery  requiring  relatively  small  starting  torque. 
329.  The  Condenser-compensator  Method  of  Starting 
Single -phase  Induction  Motors  (Fig.  214)  is  an  example  of 
the  split-phase  method  of  starting  whereby  permittance  or 
capacity  (capacitance)  is  introduced  in  the  starting  circuit. 
The  rotor  is  of  the  squirrel-cage  type.  Two  terminals  (Fig. 


Aufo- 
Tmnsfyrmer 


TMY-Connectk>n"cinct      I-"Delt«  Connection" 
Transformer  and  Auto-Transformer 


FIG.     214. — A     single-  FIG.  215. — Diagrams  of  split-phase- 
phase,  self-starting  induction  starting  single-phase  motors  ultiliz- 
motor  with  a  condenser  start-  ing  capacity  in  the  starting  circuit, 
ing  arrangement. 

215),  A i  and  B]  of  the  stator  winding,  which  is  esentially  simi- 
lar to  a  standard  three-phase  winding  are  connected  to  the 
supply  mains.  The  third  terminal,  Ci,  of  the  stator  wind- 
ing is  connected  to  the  line  through  a  transformer  as  shown 
in  7  or  through  an  auto-transformer  (or  compensator),  as 
shown  in  II.  The  main  to  which  the  lead,  LI  or  L2,  from  the 
transformer  or  auto-transformer  is  connected  is  determined 
by  the  direction  of  rotation  desired.  A  condenser  or  per- 
mitter,  Pi,  is  also  connected  across  the  auto-transformer,  as 
shown  in  the  illustration,  to  provide  permittance  or  capacity. 
The  motor  is  started  with  both  switches,  M  and  S,  closed,  but 


212 


ELECTRICAL  MACHINERY 


[ART.  330 


when  it  has  attained  a  speed  approaching  synchronism, 
the  starting  winding  is  cut  out  (by  operating  the  switch,  S) 
and  the  motor  then  continues  to  operate  on  the  running  wind- 
ing only.  This  method,  which  is  due  to  Steinmetz,  is  seldom, 
if  ever,  now  used  commercially  but  is  of  theoretical  interest. 
330.  The  Shading-coil  Method  of  Starting  Induction  Mo- 
tors is  illustrated  in  Figs.  216  and  217.  The  face  of  each 
pole  of  a  machine  which  has  been  designed  to  be  started  by 
this  method  has  arranged  in  it  a  small  short-circuited  copper- 


Shaaing 
Coils     ' 


f  FIG.  216. — Illustrating  the  applica- 
tion of  shading  coils  in  single-phase 
motors. 


FIG.      217. — Illustrating      the 
principle  of  the  shading  coil. 


bar  winding  called  a  shading  coil,  which  encircles  a  portion  of 
the  pole.  When  the  alternating  flux  passes  through  the  pole, 
it  induces  a  current  in  the  shading  soil,  S,  Fig.  217,  which,  by 
virtue  of  Lenz's  law,  tends  to  oppose  the  flux  in  portion  B, 
which  produces  it.  The  result  is  that  the  flux  in  the  ''un- 
shaded "  portion  of  the  pole,  A,  Fig.  217,  attains  its  maximum  at 
a  different  time  than  does  the  flux  in  the  "shaded"  portion  B. 
Furthermore,  as  A  and  B  are  also  space  displaced  against  each 
other,  the  result  is  that  a  field,  which  approximates  in  its  effect 
the  rotating  field  of  a  polyphase  motor,  is  produced,  but  this 


SEC.  8]         INDUCTION  AND  REPULSION  MOTORS 


213 


"shifting"  field  is  not  as  effective  as  that  developed  in  split- 
phase  machines.  The  method  has  the  further  disadvantage 
that  the  shading  coil  is  always  in  position  on  the  pole  and 
that  it  involves  an  energy  loss  so  long  as  the  motor  is  in  opera- 
tion. However,  the  method  is  applied  only  in  machines  of 
very  small  capacity,  such  as  fan  motors,  so  that  the  energy 
loss  is  not  a  matter  of  great  consequence.  The  use  of  this 
method  is  constantly  decreasing. 

331.  Performance  Data  for  Single-phase  Induction  Motors 
of  capacities  of  from  1  to  50  h.p.  will  be  found  tabulated  in 
the  author's  AMERICAN  ELECTRICIANS'  HANDBOOK.  The 


IE-  Connection  Diagram 

FIG.  218.  —  The  compensated  induction  motor  (Wagner  type  BK). 


I-  Section 
of  Slot 


efficiencies,  per  cent,  slip,  pull-out  torque  and  also  the  effi- 
ciencies and  power-factors  at  various  loads  are  there  given. 

332.  A  Compensating  Winding  on  an  alternating-current 
motor  (C,  Fig.  218)  is,  as  the  term  is  used  herein,  a  winding  the 
function  of  which  is  only  to  improve  the  power-factor  of  the 
current  taken  by  the  machine.  Thus,  the  power-factor  of  an 
alternating-current  motor,  may,  by  the  addition  of  a  suffi- 
ciently powerful  compensating  winding  be  raised  from  lagging 
to  unity  power  factor  or  to  leading  power  factor,  if  desirable. 
All  un  compensated  alternating-current  motors  draw  lagging 
current  from  the  line.  This  means  that  they  require  greater 
currents  than  are  .actually  necessary  for  the  production  of  power 


214  ELECTRICAL  MACHINERY  [ART.  333 

which  they  develop.  But  by  using  a  suitably-designed  com- 
pensating winding  it  is  possible  to  raise  the  power-factor  of  the 
motor  current  to  unity  and  thereby  proportionately  decrease 
the  current  required  for  the  production  of  the  power. 

333.  A  Neutralizing  Winding,  as  the  term  is  here  used,  is 
one  the  main  function  of  which  is  to  neutralize  armature  reac- 
tion. Hence,  if  a  neutralizing  winding  is  placed  on  a  machine 
it  will  increase  its  output  over  that  of  the  same  machine  with 
out  the  neutralizing  winding  and  incidentally  it  will  improve 
the  commutation. 


Centrifugal 
\Swltch 


L&xcfs  from— 

Compensating-  -  -  ~Y 
Brushes 


FIG.  219. — A  Wagner  compensated  induction  single-phase  motor. 

334.  The  Compensated  Induction  Motor  (classification  16, 
Table  321)  Figs.  218  and  219,  is  essentially  a  squirrel-cage- 
rotor  induction  machine  to  which  have  been  added,  on  the 
stator,  a  compensating  winding  and,  on  the  rotor,  a  direct- 
current  armature  winding  and  commutator.  Brushes  and 
connections  for  the  compensating  winding  are  provided  as 
shown  in  Fig.  218  and  also  there  are  provided  a  pair  of  short- 
circuited  brushes,  E\  and  Ez,  which  contribute  partly  to  the 
mechanical  output  of  the  machine.  These  brushes  E\  and 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS  215 

E2  and  the  connecting  bar  between  are  not  essential  to  the 
machine's  operation,  but  they  increase  its  efficiency.  The 
feature  of  machines  of  this  type  that  has  brought  them  into 
use  is  that  they  operate  with  good  power-factor  under  all  load 
conditions.  They  have  the  further  advantage  that,  except  in 
starting,  practically  all  of  the  work  is  done  by  virtue  of  the 
squirrel-cage  winding;  hence,  the  brushes  and  commutator 
carry  little  load  current,  commutation  difficulties  are  mini- 
mized, the  life  of  the  commutator  is  increased  and  its  size 
decreased. 

The  squirrel-cage  winding  consists  of  copper  bars,  C  (Fig. 
218,  //)  wedged  in  the  bottoms  of  the  slots  in  the  rotor.  A 
"magnetic  bridge,"  I,  of  iron,  is  arranged  above  the  squirrel- 
cage-bar  winding  and  then  the  commutating  winding — similar 
to  a  direct-current  armature  winding — is  placed  in  the  slot 
above  the  bridge.  Fig.  218  shows  the  connections  as  they 
exist  after  the  machine  has  attained  running  speed.  While 
the  machine  is  starting,  the  compensating  winding  is  not  in 
use  because  its  circuit  is  then  held  open  by  a  centrifugal 
switch  which  is  located  within  the  casing  5,  Fig.  219.  These 
machines  as  manufactured  by  the  Wagner  Electric  Manu- 
facturing Company  have  a  starting  torque  equal  to  1J^  to  2 
times  full-load  torque.  Their  starting  current  is  about  3  times 
full-load  current.  When  the  machine  is  in  running  connection, 
the  full-load  power-factor  is  about  unity.  The  adjustment  of 
the  compensating  winding  is  so  made  that  at  full-load  the 
power-factor  is  approximately  unity,  while  at  no-load  it  is 
usually  appreciably  leading. 

335.  The  Straight  Repulsion  Motor  (classification  2c,  Table 
321)  has,  as  illustrated  in  Fig.  220,  an  armature  which  is  like 
that  of  a  direct-current  machine.  Short-circuiting  brushes, 
EI  and  E2,  inclined  at  an  angle  to  the  axis  of  the  stator  winding 
are  provided.  A  machine  of  this  type  has  in  general  the  same 
speed  torque  characteristics  as  a  direct-current  series  motor, 
that  is,  high-starting  torque  with  a  very  small  starting  current 
and  rapidly  decreasing  torque  with  increasing  speed.  The 
power-factor  of  this  machine  increases  with  increasing  speed 
and  near  synchronous  speed  attains  a  value  which  is  higher 


216 


ELECTRICAL  MACHINERY 


[ART.  336 


than  usually  obtained  in  straight  induction  motors.  These 
motors  are  used  principally  for  constant-torque  applications, 
namely,  for  printing-press  drives  and  also  for  other  drives  for 
which  series  direct-current  motors  are  adaptable,  such  as  for 
fans  and  blowers.  They  have  been  successfully  applied  for 
hoist  service.  As  with  the  series  direct-current  motor,  the 
speed  of  these  machines  can  be  varied  by  varying  the  im- 
pressed voltage  or  by  shifting  the  brushes.  The  direction  of 
rotation  of  a  straight  repulsion  motor  can  be  changed  by  the 
addition  of  a  suitable  reversing  switch  or  reversing  winding  or 
by  shifting  the  brushes  to  the  reverse  side  of  the  neutral. 


FIG.  220. — Repulsion  motor. 

336.  The  Repulsion-induction  Motor  (classification  2d, 
Table  321),  Figs.  221  and  222,  is  like  the  straight  repulsion 
machine  except  that  in  addition  it  has  a  compensating  winding 
as  shown  in  the  diagram.  The  energy  brushes,  EI  and  E2, 
have  approximately  the  same  angular  position  in  relation  to 
the  stator  winding  as  the  energy  brushes  of  a  straight  repul- 
sion motor.  These  machines  have,  in  addition  to  the  energy 
brushes,  a  second  set  of  brushes,  Ci  and  Cz,  called  the  com- 
pensating brushes,  which  are  connected  in  series  with  the 
compensating  winding.  The  motors  have  a  starting  torque 
equal  to  about  2K  to  3  times  full-load  torque  with  ap- 
proximately twice  full-load  current.  The  maximum  torque 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS 


217 


is  from  3  to  3}£  full-load  torque.     The   power-factor    (due 
to  the  corrective  action  of  the  compensating  winding)  is  very 


Main 
Winding 


Frame-*  ~"       . "  '  r>.""  E-Llements 

I- Connection  Diagram 

FIG.  221. — Diagrams  of  the  repulsion  induction  single-phase  motor. 
(The  brushes  2?8  and  .#4  in  //  should  be  slightly  inclined  in  the  clockwise 
direction  from  the  vertical,  instead  of  being  in  a  vertical  line  as  shown.) 


Spiral  Slotting 

of  Armature 


Commutator-. 


Bearing 


FIG.  222. — Phantom  view  showing  construction  of  the  General  Electric 
Company  Type  RI,  Form  C,  single-phase,  repulsion-induction  motor. 

high  at  all  loads  but  the  efficiency  of  this  machine  is  lower 
than  that  of  the  induction  motor.  Single-phase  motors  of  this 
type  are  well  adapted  for  loads  involving  heavy  starting  torque 


218 


ELECTRICAL  MACHINERY 


[ART.  336 


and  sudden  overloads.  They  have  the  disadvantages  of  hav- 
ing commutators  which  carry  the  total  armature  current. 
They  can  be  arranged  for  variable  speed  service  as  shown 


•Insulation 


./'Resistance 


I- Connect  ion  Diagram  I- Elements 

FIG.  223. — Variable-speed  repulsion-induction,  single-phase  motor  and 

controller.  • 

in  Fig.  223  by  the  insertion  of  a  rheostat  in  series  between 
the  energy  brushes.  They  can  also  be  arranged  for  reversing 
service  as  indicated  in  Fig.  224  by  the  application  of  a  revers- 


Compensating  Field 
I- External  Connections 


Reversing  Switch-— J 
I-Elements 


FIG.  224. — A  repulsion-induction  motor  arranged  for  reversing  service. 
(Throwing  the  reversing  switch:  (1)  reverses  the  connection  between  the 
compensating  winding  and  the  compensating  brushes,  (2)  reverses  the  con- 
nections between  the  reversing  field  winding  and  the  main  winding.) 

ing  field  and  a  suitable  reversing  switch.  The  characteristic 
performance  graphs  of  machines  of  this  type  are  shown  in 
Fig.  225. 


SEC.  8] 


INDUCTION  AND  REPULSION  MOTORS 


219 


337.  If  a  Variable -speed  Single-phase  Motor  is  Required, 

some  modified  form  of  repulsion-induction  motor  (Fig.  223) 
can  be  used.  Also,  the  series  motor  (Fig.  233)  and  the  re- 
pulsion motor  (Fig.  220)  may  be  applied  for  this  service  by 
using  a  rheostat  or  an  auto-transformer  in  the  line  for  vary- 
ing the  voltage  impressed  on  the  motor  terminals.  The 
behavior  of  such  motors  is  similar  to  that  of  the  variable-speed 
wound-rotor  multiphase-induction  motor  with  resistance  in 
series  with  the  rotor.  They  are  consequently,  owing  to  their 
unstable  speed  characteristics,  suited  only  to  such  applications 


0  25  50  75 

Per  Cent  Syn.  Speed 
I- Typical  Speed  Torgue  Curves (Z20V) 


50          75          100         l?5        150 
Per  Cent  Load 

tt-Eff icienc-y.Pwer-Factor  and  Speed 


FIG.  225. — Typical  performance  graphs  for  a  single-phase,  repulsion- 
induction  motor.     (General  Electric  Co.,  Type  RI,  Form  C.) 

as  require  a  steady  horse-power  at  given  speeds.  Their 
characteristics  as  regards  starting  torque,  etc.,  are  unchanged 
when  used  for  variable  speeds.  The  speed-regulating  resist- 
ance (R,  Fig.  223)  is  in  one  type  of  motor  inserted  in  series 
with  the  brushes  which  are  normally  short-circuited  and  the 
insertion  of  additional  resistance  decreases  the  speed. 

338.  Repulsion-starting-and-Induction-running,  Single- 
phase  Motors  (classification  3,  Table  321),  Fig.  227,  are  now 
probably  used  for  power  service  to  a  greater  extent  than 
those  of  any  other  type.  As  would  be  inferred  from  its  name 
(Fig.  228)  the  motor  starts  as  a  repulsion  but  operates  as  an 
induction  machine.  The  rotor,  Fig.  229,  is  exactly  the  same 


220 


ELECTRICAL  MACHINERY 


[ART.  338 


I 

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200         600          1000        1400        1800 
400         600         1200         1600       2000 

Rev.  Per  Min. 

I-  3  h.p.  Motor  Operating  Under  Constant 
Full-Load  Torque  at  Different 


•  400   m  800   m  1200  14°°, 600 %C 

Rev.  Per  Min. 

1-3  h.p.  Motor  Operating  Under  Constant 
Full-Load  Torgue  at  all  Speeds 
(Voltage  at  Motor  Terminals  Varied  by 
Means  of  Auto-Transformer  as  in  I) 


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100  200 

HI- Starting  Characteristic 

(Variable-Speed  Motor  Operating  with  Standard  Controller 
with  Auto-Transformer  Voltage  Supplied  to  Motor  Varied  with 
Auto-Transformer) 


15    20    25    30    35    40    45    50 
JY-Performance  Under 
Variable-Torque  Conditions 
'(Voltage  Supplied  to  Motor 
Maintained  Constant  at  110 
Volts.  Line  Voltage  on  Autp- 
Transformer  was  220 
During  Test) 


.-Arm  for  Shifting 
'  Brushes 


Y-  Cylinder-Press  Victor 

(With  Auto-Transformer  Control ler) 


3ZH-  Application  of  Job  Press  Control 


W-Adjustable-Speed  Motor 


fl    rt    4 


,  226.— See  explanation  on  following  page. 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS  221 

as  the  armature  of  a  direct-current  motor.  It  is  provided 
with  form-wound  coils  and  a  commutator.  The  number  of 
brushes  used  is  the  same  as  would  be  used  with  the  corre- 
sponding direct-current  motor  with  the  same  number  of  poles, 


'*•*•  Slide  Ret  Us '' 


FIG.  227. — Showing  the  external  appearance  of  the  Wagner,  Type  BA, 
repulsion-starting-and-induction-running  motor. 

FIG.  226. — Kimble  single-phase  motors.  In  V  on  preceding  page 
is  shown  the  control  used  for  the  cylinder-printing-press  motors.  By 
using  two  control  handles  and  an  auto-transformer,  30  over-lapping 
speed  steps  are  obtained.  The  handles,  as  shown  in  the  picture,  are  in 
the  position  of  maximum  speed,  impressing  110  volts  on  the  motor. 
By  utilizing  the  auto-transformer  control,  the  same  outfit  can  be  used 
either  on  110  or  220-volt  circuits.  On  a  220-volt  circuit,  the  line  wires 
are  connected  to  terminals  LI  and  L2  and  on  a  110-volt  circuit  to  T 
and  6.  This  motor  (and  also  that  shown  in  VIII  use  a  fixed-brush 
setting.  That  is,  the  brushes  are  not  shifted  for  speed  control.  The 
motor  of  V  and  also  that  of  VI  can  be  used  _  only  on  constant-torque 
loads.  In  VI  is  shown  the  ideal  diagram  for  the  job-printing-press  motors, 
while  in  VII  is  illustrated  the  method  of  their  application.  Speed  con- 
trol is  obtained  by  shifting  the  brushes.  In  VIII  the  adjustable-speed 
motor  is  shown.  The  speed  is  controlled  with  an  auto-transformer 
whereby  the  e.m.f.  impressed  on  the.  line  terminals  M  and  N  is  varied. 
The  speed  is  maintained  constant  at  any  given  speed  within  the  range  of 
the  motor  by  the  action  of  the  centrifugal  governor  G  which  opens  and 
closes  the  main  circuit,  motors  of  this  type  are  made  in  capacities  of  1.6 
to  2.5  h.p.  All  of  the  Kimble-Company  motors  are  of  the  compensated- 
series  type  and  employ  a  neutralizing  winding  to  neutralize  armature 
reaction  N.  This  winding  also  improves  the  power-factor. 


222 


ELECTRICAL  MACHINERY 


[ART.  338 


but  in  the  alternating-current  machines  all  of  the  brushes  are 
connected  together  electrically  by  the  metal  rocker  arm  which 
supports  them.  The  stator,  which  has  only  one  winding,  is 


Stator 


Armature   Similar 
to  a  D.  C.  Armature 


Commutator  Bars 
Connected  with  a 
Short -Circuiting  Ring. 


I~  Star  ting 
(Starts  an  a  Repulsion  Motor) 


^Brushes  Bearing 
On  Commutator 


Stator  'Winding 

Brushes  Raised    "\ 
from  Commutator' 
H-  Running 
(Runs  as  an  Induction  Motor) 


FIG.  228. — Illustrating  the  principle  of  the  repulsion-starting,  induction- 
running,  single-phase  motor. 

supplied   with  single-phase  current.     There  is  no  electrical 
connection  (Fig.  228)  between  the  stator  and  the  rotor.     The 


•Eye  Bolt 
Motor  Frame 


Short 
Circuiting 
Weights  -^ 


Circuiting 
Ring 

Commutator- 
Insulation 
Brush  Holder  ••' 
Knock  Off  Piece 


FIG.  229. — Sectional  elevation  of  the  Wagner  Type  BA  repulsion-start- 
ing-and-induction-running  motor. 

currents  in  the  stator  create  an  alternating  flux  which  reacts 
on  the  rotor  and  induces  its  rotation.  When  the  speed  of  the 
rotor  approaches  synchronous  speed,  a  centrifugal  device  of 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS 


223 


Brush— -> 


^'Commutator  Bar 
Brush  Holder  Spring 


U- Motor  at  Full  Speed 

FIG.  230. — Illustrating  the  mechanism  of  the  brush-lifting  and  commu- 
tator short-circuiting  device  of  the  Wagner  repulsion-starting-and-induc- 
tion-running  motor. 


224 


ELECTRICAL  MACHINERY 


[ART.  339 


some  description  short-circuits  the  commutator  bars  (Fig.  228, 
II  and  Fig.  230)  and  simultaneously  pushes  the  brushes  away 
from  the  commutator.  Thus  the  motor  is  transformed  into 
an  induction  machine  having  what  is  essentially  a  squirrel- 
cage  rotor.  Typical  performance  graphs  are  shown  in  Fig. 
23 1 .  The  machine  is  inherently  a  constant-speed  motor.  The 
applications  of  the  repulsion-starting-and-induction-running 
motor  are  about  the  same  as  those  for  which  a  direct-current 
constant-speed  shunt  motor  is  ordinarily  used  with  the  excep- 
tion that  the  repulsion  induction  motor  has  a  much  greater 


1800 


FIG.  231. — Typical  characteristic  curves  of  the  repulsion-starting, 
induction-running  motor  of  the  general  type  illustrated  in  Fig.  228 
(Peerless  Electric  Co.). 

starting  torque  (Fig.  232)  than  the  corresponding  shunt  motor 
and  that  it  is  not  adaptable  for  adjustable-speed  service. 

339.  The  Series  Single-phase  Motor  (classification  4,  Table 
321)  comprises,  Fig.  233,  merely  an  armature  of  the  same 
construction  as  any  direct-current  armature  in  series  with  a 
field  winding.  In  fact,  any  direct-current  series  motor  will 
operate  on  single-phase  alternating  current  but  if  the  machine 
is  of  such  a  capacity  that  it  requires  considerable  current, 
commutation  difficulties  will  be  encountered.  Furthermore, 
for  alternating-current  work  the  stator  core  should  be  lami- 


SEC.  8]          INDUCTION  AND  REPULSION  MOTORS 


225 


s|ue  and  Current 

i  i  \ 

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X 

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•Without 

'Starting- 

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art  ing  & 

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FIG.  232. — Typical  starting  torque  and  starting  current  graphs  for 
repulsion-starting,  induction-running,  single-phase  motors  (Century  Elec. 
Co.).  NOTE. — Motors  of  standard  construction  and  rating  will  be  found 
capable,  when  the  voltage  and  frequency  for  which  they  are  wound  and 
adjusted  is  maintained,  of  bringing  a  load  up  to  speed  equal  to  at  least 
1^  of  their  rated  capacity.  When  connected  directly  across  the  line 
(which  is  the  usual  manner  of  making  the  installation),  they  will  develop, 
at  the  moment  of  starting,  approximately  two  and  one-half  times  full- 
load  torque  and  take  approximately  two  and  one-half  times  full-load 
current.  By  the  time  the  motor  has  reached  one-quarter  speed,  the 
torque  will  have  increased  to  approximately  five  times  full-load  torque 
and  the  current  decreased  to  double  full-load  current;  from  that  speed 
both  torque  and  current  gradually  drop  to  normal  as  the  motor  increases 
in  speed.  By  the  use  of  a  resistance  starter,  full-load  starting  torque 
may  be  secured  with  approximately  one  and  one-quarter  times  full-load 
current  under  the  most  favorable  conditions.  In  commercial  practice 
the  starting  current  may  be  expected  to  be  around  one  and  one-half 
times  full-load  current. 


Armature. 


Winding 


Cut  Out^ 


?j\- -Brushes 


sFieia  Winding  *Swifcfl 


-Armature 


E-Elements 


l-^onnect ion  Diagram 


FIG.  233. — Diagrams  of  the  series,  single-phase  induction  motor.     (The 
so-called  "universal"  motor.) 


15 


226 


ELECTRICAL  MACHINERY 


[ART.  340 


Wall- 


'Single  -Phase  Motor          C 
\  Efficiency  at  this  Load=80%^. 


nated.  However,  for  motors  of  small  capacity  where  the 
currents  are  of  low  intensity,  the  commutation  difficulties  are 
not  unsurmountable,  hence  small  single-phase  series  motors 
are  being  manufactured  and  used  successfully  in  capacities  of 
less  than  1  h.p.  The  important  commercial  example  of  this 
type  of  motor  is  the  so-called  " universal"  motor  which  will 
operate  on  either  direct-  or  alternating-current  circuits.  It 

is  used  for  fans  and  small  appli- 
ance motors  and  electric  drills, 
where  the  manufacturers  in  send- 
ing out  the  motor  may  not  know 
whether  the  ultimate  purchaser 
will  have  available  either  alter- 
nating or  direct  current.  These 
are  variable-speed  machines, 
that  is,  they  have  (approxima- 
tely) the  speed-torque  charac- 
teristics of  a  direct-current  series 
motor. 

340.  To  Compute  Either  the 
Horse-power,  Current,  Voltage, 

FIG.  234.-Example  in  com-  Power-factor  or  Efficiency  of 
puting  the  horsepower  output  of  any  Single-phase  Alternating- 
a  single-phase  motor.  current  ^^  the  Qther  quanti_ 

ties  being  known,  one  of  the  following  formulas  may  be  used : 
E   X  Ii   X  p.f.    X  E 


(69) 
(70) 
(71) 
(72) 
(73) 


746 


JU 

it 

p-f- 

T? 

/i  X 

h.p. 

p.f.  X  E 
o  X  746 

E  X 
h.p.c 

p.f.  X  E 
,X  746 

EX 

h.p 

IiXE 
.0  X  746 

(horse-power) 
(volts) 
(amperes) 
(power-factor) 
(efficiency) 


p.f.  X  E  X  Ii 

Wherein,    h.p.0  =  power  output  of  the  motor,  in  horse-power. 
E  =  alternating  voltage  impressed  between  the  wires  on  motor 


SEC.  8]         INDUCTION  AND  REPULSION  MOTORS 


227 


terminals,  in  volts.  I\  —  current  in  each  of  the  two  wires  of 
the  motor  in  amperes,  p.f.  =  power-factor  of  the  motor,  ex- 
pressed decimally.  E  =  efficiency  of  the  motor,  expressed 
decimally. 


,-'2200-Vo/t  Prim 


^220-Volt  Secohdaru 


Single  -Phase 
1  10  -Volt 
Alternating- 
Current  Motor—*''' 


Two- -Phase  --' 
220-Volt 
Alternating- 
Current  Motor 


FIG.  235.  —  Diagram  illustrating  methods  of  feeding  motors  from  a  two- 

phase  line. 

EXAMPLE.  —  What  will  be  the  horse-power  output  of  the  pulley,  P,  of 
the  single-phase  motor  shown  in  Fig.  234,  when:  the  impressed  voltage 
is  220,  the  current  is  10  amp.,  the  power-factor  is  84  per  cent,  and  the 
efficiency  is  80  per  cent.?  SOLUTION.  —  Substitute  in  the  formula  (69): 


j»  -  -2200- Volt-  Primary 


•220-Volt  Secondary 


Single- Phase 
1 10 -Volt 
Alternating- 
Current  Motor. 


Single -Phase 
1 10 -Volt 
Alternating- 
Current  Motor--,. 


Single-Phase- 
220-Volt 
Alternating- 
Current  Motor 


Three-Phase-' 
220-Volt 
Alternating- 
Cvrrent  Motqr 


FIG.  236. — Showing  single-phase  motors  fed  from  a  three-phase  line. 

h.p.0  =  (E  X  1 1  X  p.f.  X  E)  -5-  746  =  (220  X  10  X  0.84  X  0.80)  -s-  746 
=  1,478.4  -J-  746  =  1.98  h.p.,  or,  say,  2  h.p. 

EXAMPLE. — What  will  be  the  full-load  current  taken  by  a  10-h.p.  single- 
phase,  220-volt  induction  motor,  which,  at  its  rated  horse-power  has  a 
power-factor  of  80  per  cent.,  and  an  efficiency  of  84  per  cent.?  SOLU- 
TION.— Substitute  in  equation  (71):  /i  =  (h.p.0  X  746)  -f-  (E  X  p./.  X 
E)  =  (10  X  746)  -v-  (220  X  0.80  X  0.84)  =  7,460  +  147.84  =  50.4  ampt 


228  ELECTRICAL  MACHINERY  [ART.  341 

341.  That  Single-phase  Motors  may  be  Operated  from  any 
Alternating-current  Circuit  is  Their  Principal  Advantageous 
Feature. — Obviously  they  may  be  operated  from  a  single- 
phase  circuit  and  can  also  be  operated  from  two-phase  and 
three-phase  circuits,  as  diagrammed,  respectively,  in  Figs.  235 
and  236.  Frequently  utility  companys  do  not  extend  poly- 
phase circuits  to  certain  outlying  districts.  Where  a  motor  is 
to  be  installed  in  such  a  territory  and  the  extension  of  the 
polyphase  circuit  is  not  economically  desirable,  a  single-phase 
motor  offers  a  solution  to  the  problem. 


SECTION  9 
SYNCHRONOUS  MOTORS  AND  CONDENSERS 

342.  Synchronous  Motors*  (Fig.  237  and  238).— Generally 
speaking,  any  modern  alternating-current  generator  will  oper- 


Position  I 


Position  II 


Position  H  Position  ET 

FIG.  237. — The  operating  principle  of  the  synchronous  motor. 

ate  with  more  or  less  satisfaction  as  a  synchronous  motor, 
and  unless  special  operating  features  must  be  provided  for, 
the  two  are  often  identical  in  construction.  There  are  two 

*  Carl  C.  Knight  in  PRACTICAL  ENGINEER,  June,  1  1912. 

229 


230 


ELECTRICAL  MACHINERY 


[ART.  342 


advantages  of  the  synchronous  motor,  namely:  it  operates  at 
a  constant  speed  at  all  loads,  provided  the  driving  alternator 
runs  at  a  constant  speed,  and  its  power-factor  is  at  all  times 
under  the  control  of  the  attendant.  It  can  be  used  to  correct 
low  power-factor  of  the  system  that  feeds  it,  in  addition  to 
driving  a  mechanical  load,  provided  it  has  sufficient  capacit}r. 
The  latter  characteristic  is  often  of  considerable  impor- 
tance. It  is  well  known  that  the  power-factor  of  the  induc- 
tion motor,  even  under  full-load  conditions,  is  seldom  greater 


FIG.  238. — A  Westinghouse  Type  C  belt-drive  synchronous  motor  with 
a  direct-connected  exciter. 

than  95  per  cent.,  and  it  often  falls  as  low  as  50  or  60  per 
cent,  at  light-load.  The  result  is  that  an  alternating-current 
generator  driving  a  considerable  number  of  induction  motors 
ordinarily  operates  at  a  comparatively  low  power-factor.  If 
this  alternator  is  loaded  to  its  full  kilowatt  capacity  at  such  a 
low  power-factor,  overheating  will  result.  If  the  alternator 
is  not  loaded  beyond  its  normal  current  capacity  it  operates 
at  a  low  energy  load  but  with  the  same  heating  losses  as  at 
full-load,  on  account  of  the  reduced  power-factor.  The  ad- 
vantage of  the  synchronous  motor  on  such  a  system  is,  that 


SEC.  9]    SYNCHRONOUS  MOTORS  AND  CONDENSERS         231 

by  proper  adjustment  of  its  field  current  it  may  be  made  to 
draw  from  the  line  a  current  which  is  leading  with  respect  to 
the  voltage.  This  current  which  will  " neutralize"  the  lagging 
current  taken  by  the  induction  motors.  The  current  in  the 
alternating-current  generator  can  thereby  be  brought  into  phase 
with  the  voltage  and  the  generator  will  operate  under  its 
normal  conditions.  When  used  in  this  manner  as  a  com- 
pensator for  lagging  current,  the  synchronous  motor  must  be 
of  larger  size  than  required  by  its  mechanical  power  output, 
on  account  of  the  excess  current  which  it  draws  from  the  line. 

343.  A  Synchronous   Condenser  is  a  synchronous  motor 
that  operates  to  correct  power-factor  only  and  does  not  pull 
any  mechanical  load. 

344.  Disadvantages  of  the  Synchronous  Motor. — To  offset 
its  advantages,   the   synchronous  motor  has   disadvantages 
which    ordinarily  limit    its    application    to    relatively  large 
capacities,  and  to  installations  where  it  can  be  used  as  a 
"  neutralizer  "  for  lagging  current.     The  chief  disadvantage 
is  that  the  motor  has  relatively  small  starting  torque  even 
at  full-load  current.     The  motor  also  requires  direct  current 
for  its  field  excitation. 

345.  The  Uses  of  Synchronous  Motors. f — Due  to  the  fact 
that  synchronous  motors  require  more  care  than  induction 
motors,  that  they  are  not  self-exciting  and  are  started  with 
some  difficulty,  they  are  seldom  employed  where  induction 
motors  can  be  used.     Where  an  induction  motor  would  be 
objectionable  on  account  of  its  lagging  " wattless"  currents 
which  affect  the  voltage  regulation,  a  synchronous  motor  may 
be  used  to  advantage.     It  is  also  used  as  a  "  synchronous  con- 
denser" in  connection  with  induction-motor  loads  for  power- 
factor  correction  as  noted  above. 

346.  The  Steps  in  Starting  a  Synchronous  Motor  are  about 
as  follows: 

1.  See  that  motor  is  clean,  that  bearings  are  well  supplied 
with  oil,  and  that  oil  rings  are  free  to  turn. 

2.  See  that  all  switches  are  open. 

3.  Close  the  double-throw  field  switch,  cutting  in  the  field 
rheostat  with  its  resistance  all  in. 

t  STANDARD  HANDBOOK. 


232 


ELECTRICAL  MACHINERY 


[ART.  347 


_       Incoming  Line 

Voltage  &J&  ' 
Tronsforffiot  lii 


Power  Factor  Meter 
Ammeter 


4.  Close  the  main-line  switch  (if  any)  in  the  circuit  and 
throw  in  the  double-throw  switch,  throwing  it  in  the  starting 
position.     The  motor  should  start  and  speed  up  to  synchron- 
ism in  from  30  to  60  sec. 

5.  When  motor  is  up  to  speed,  throw  field  switch  over  to 
the  other  (running)  position  with  rheostat  all  in. 

6.  Throw  double-throw  main  switch  over  to  running  posi- 
tion, putting  motor  on  full-line  voltage. 

7.  Adjust  field  rheostat  for  minimum  armature  current. 
Fig.  239  shows  the  method  of  connecting  a  three-phase, 

self-starting  synchronous  motor  to  its  exciter.     This  diagram 

shows  a  double-throw  switch  in 
the  field  circuit.  This  switch, 
however,  may  (where  the  exciter 
is  connected  to  the  same  shaft 
Excifffr^M  as  the  synchronous  motor)  be 
single-throw  and  the  field  con- 
nected direct  through  the  exciter 
armature  with  the  rheostat  in  the 
circuit.  The  field  is  thus  short- 
circuited  at  standstill  and  is 
gradually  charged  as  the  motor 
speeds  up. 

347.     Starting     Synchronous 

Motors.  * — Practically  any  poly- 
FIG.  239. — Connections  for  a  self-     ,  i 

starting  synchronous  motor.      phase   synchronous  motor  may 

be  started  by  applying  full-load 

voltage  to  the  armature,  leaving  the  field  open  until  the  motor 
has  reached  its  normal  speed.  Such  a  procedure  would  require, 
however,  2  or  more  times  the  full-load  current  of  the  machine. 
Since  the  power  taken  by  a  synchronous  motor  starting  in  this 
manner  is  of  very  low  power-factor,  the  line  disturbances 
might  be  considerable.  Starting  at  full-line  voltage  is  also 
liable  to  induce  in  the  field  windings  an  excessively  high  vol- 
tage, often  resulting  in  breaking  down  the  insulation. 

348.  To  Limit  the  Starting  Current  to  a  Reasonable  Value, 
Auto -starters  or  Compensators  are  Often  Used. — These  are 

*  PRACTICAL  ENGINEER. 


SEC.  9]    SYNCHRONOUS  CONDENSERS  AND  MOTORS          233 

similar  and  used  in  exactly  the  same  manner  as  the  starting 
compensators  used  with  induction  motors  (Art.  371).  When 
starting  with  a  compensator,  the  field-winding  circuit  is  opened 
by  a  switch  provided  for  the  purpose  or  the  field  circuit  may 
be  closed  through  a  resistance  until  the  motor  has  attained  its 
normal  speed.  This  arrangement  does  not  provide  a  great 
starting  torque,  and  in  most  modern  synchronous  motors  the 
revolving  field  of  the  motor  is  provided  with  a  special  auxiliary 
winding  (Fig.  240)  similar  to  the  winding  on  the  rotor  of  a 
squirrel-cage  induction  motor.  It  has  been  possible  to  con- 
struct motors  having  nearly  30  per  cent,  of  full-load  torque 
at  approximately  1%  times  full-load  current.  Beside  improv- 


Pole 


FIG.  240. — Rotating   element   of   a   synchronous   motor   showing   the 
squirrel-cage  starting  winding. 

ing  the  starting  torque  this  squirrel-cage  winding  also  has  a 
tendency  to  reduce  the  hunting  or  pumping  effect  which  is 
sometimes  encountered  in  the  operation  of  synchronous  motors. 

349.  Where  the  Capacity  of  the  Motor  Which   is  to  be 
Started  is  Comparable   with  the  Capacity  of  the  Generator 
which  feeds  it,  it  is  often  necessary  to  connect  a  small  induc- 
tion motor  to  the  synchronous  motor  to  bring  it  up  to  speed. 
When  approximately   normal    speed   has   been   reached  the 
synchronous  motor  is  thrown  on  the  line  as  before,  and  the 
field  closed  immediately. 

350.  When  a  Large  Starting  Torque  is  Required,  as,  for 
example,  in  driving  a  considerable  amount  of  shafting,  it  is 


234  ELECTRICAL  MACHINERY  [ART.  351 

often  impractical  to  start  the  load  and  the  motor  from  rest 
simultaneously.  In  such  instances  it  is  customary  to  install 
a  friction  clutch  or  similar  device  between  the  motor  and  its 
load,  so  that  the  motor  may  attain  its  normal  speed  before 
any  load  is  imposed  upon  it. 

351.  Occasional  Installations  are  Encountered  Where  the 
Motor  is  the  Only  Load  on  the  Driving  Generator. — In  such 
cases  it  is  possible  to  connect  the  synchronous  motor  to  the 
line  before  starting  the  alternator.     On  starting  the  alternator, 
both  will  come  up  to  speed  together.     Cases  have  been  known 
in  which  the  motor  was  a  small  part  of  the  load  on  the  driv- 
ing alternator,  that  is,  the  alternator  was  larger  compared 
with  the  motor,  when  an  auto-starter  was  used  to  raise  the 
voltage  at  start  instead  of  to  reduce  it.     This  method  gives  a 
fairly  good  torque,  but  requires  large  current,  and  the  operator 
must  be  certain  that  the  motor  windings  will  not  be  damaged 
before  trying  such  a  method. 

352.  In  Cases  Where  it  is  Desired  to  Use  an  Alternating- 
current  Generator  as  a  Motor  and  no  Compensator  is  Avail- 
able, water  rheostats  can  be  used  to  good  advantage,  one  be- 
ing placed  in  series  with  each  phase.     They  are  short-circuited 
when  the  motor  has  attained  normal  speed. 

353.  General  Summary  of  Synchronous-motor  Troubles. — 
Failure  of  a  synchronous  motor  to  start  is  often  due  to  faulty 
connections  in  the  auxiliary  apparatus.     These  should  be  care- 
fully inspected  for  open  circuits  or  poor  connections.     An  open 
circuit  in  one  phase  of  the  motor  itself,  or  a  short-circuit  will 
prevent  the  motor  from  starting.     Most  synchronous  motors 
are  provided  with  an  ammeter  in  each  phase,  so  that  the  last 
two  causes  can  be  determined  from  their  indications — no  cur- 
rent in  one  phase  in  case  of  an  open  circuit,  and  excessive 
current  in  case  of  a  short-circuit.     Either  condition  will  usually 
be  accompanied  by  a  decided  buzzing  noise,  and  in  case  of  a 
short-circuited  coil,  it  will  often  be  quickly  burned  out.     The 
effect  of  a  short-circuit  is  sometimes  caused  by  two  grounds 
on  the  machine.     Starting  troubles  should  never  be  assumed 
until  a  trial  has  been  made  to  start  the  motor  light,  that  is, 
with  no  load  except  its  own  friction.     It  may  be  that  the  start- 
ing load  is  too  great  for  the  motor. 


SEC.  9]    SYNCHRONOUS  MOTORS  AND  CONDENSERS          235 

354.  If  the  Motor  Starts  but  Fails  to  Develop  Sufficient 
Torque  to  Carry  its  Load  when  the  field  circuit  has  been  closed, 
the  trouble  will  usually  be  found  in  the  field  circuit.     First, 
determine  whether  or  not  the  exciter  is  developing  its  normal 
voltage.     Assuming  the  exciter  voltage  to  be  correct,  the  trouble 
will  probably  be  due  to  one  of  the  following  causes.     (1)  Open 
circuit  in  the  field  winding  or  rheostat  or  (2)  short-circuit  or 
reversal  of  one  or  more  of  the  field  spools.     Open  circuit  can 
often  be  located  by  inspection  or  by  use  of  the  magneto. 

355.  The  Majority  of  Field  Troubles  are  caused  by  excessive 
induced  voltage  at  start,  or  by  the  field  circuit  being  broken. 
This  excessive  voltage  may  break  down  the  insulation  between 
field  winding  and  frame  or  between  turns  on  any  one  field  spool, 
thus  short-circuiting  one  or  more  turns,  or  it  may  even  burn  the 
field  conductor  off,  causing  an  open  circuit. 

356.  Causes  of  Overheating  in  Synchronous  Motors  are 
about  the  same  as  those  in  alternating-current  generators. 
Probably  the  most  common  cause  of  overheating  is  excessive 
armature  current  due  to  an  attempt  to  make  the  motor  carry 
its  rated  load,  and  at  the  same  time  compensate  for  a  power- 
factor  lower  than  that  for  which  it  was  designed.     If  the 
motor  is  not  correcting  low  power-factor,  but  doing  mechanical 
work  only,  the  field  current  should  be  adjusted  so  that  the 
armature  field  is  a  minimum  for  the  average  load  that  the 
motor  carries. 

357.  Difficulties  in  Starting  Synchronous  Motors.  * — A  syn- 
chronous motor  is  "weaker"  in  starting  than  is  an  induction 
motor.     In  general,  however,  a  synchronous  motor  will  start 
itself  and  perhaps  a  very  light  load.     Starting  requires  no  field 
current  as  the  flux  which  tends  to  start  the  motor  is  not  the 
flux  that  operates  it  when  it  is  up  to  speed.     In  starting,  the 
field  current  is  lagging,  and  a  lagging  current  tends  to  pull 
down  the  voltage  on  the  supply  circuit,  hence  tends  to  lower 
the  applied  voltage.     The  starting  torque,  as  in  an  induction 
motor,  is  proportional  to  the  square  of  the  applied  voltage. 
For  example,  if  the  voltage  is  halved,  the  starting  effort  is 
quartered.     When  a  synchronous  motor  will  not  start,  it  may 

*  Based  largely  on  Raymond's  MOTOR  TROUBLES. 


236  ELECTRICAL  MACHINERY  [ART.  357 

be  because  the  voltage  on  the  line  has  been  pulled  down  below 
the  value  necessary  for  starting. 

In  general,  at  least  half  voltage  is  required  to  start  a  syn- 
chronous motor.  Difficulty  in  starting  may  also  be  caused 
by  an  open  circuit  in  one  of  the  lines  to  the  motor.  Assume 
the  motor  to  be  three-phase.  If  one  of  the  lines  is  open  the 
motor  becomes  single-phase,  and  no  single-phase  synchronous 
motor,  as  such,  is  self-starting.  The  motor  will,  therefore, 
not  start,  and  will  soon  get  hot.  The  same  condition  is  true 
of  a  two-phase  motor,  if  one  of  the  phases  is  open-circuited. 

Difficulty  in  starting  may  also  be  due  to  a  rather  slight 
increase  in  static  friction.  It  may  be  that  the  bearings  are 
too  tight,  perhaps  from  cutting  during  the  previous  run. 
Excessive  belt  tension,  in  case  the  synchronous  motor  is  belted 
to  its  load,  or  any  cause  which  increases  starting  friction  will 
probably  give  trouble.  Difficulty  in  starting  may  be  due  to 
field  excitation  being  on  the  motor.  After  excitation  exceeds 
one-quarter  normal  value,  the  starting  torque  is  influenced. 
With  full  field  on,  most  synchronous  motors  will  not  start 
at  all.  If  the  proper  voltage  is  applied  to  a  motor,  and  the 
circuits  are  all  closed  except  the  field  circuit  and  the  friction 
is  a  minimum,  and  still  the  motor  will  riot  start,  the  fault  is 
probably  with  the  manufacturer.  Pole-pieces  often  receive 
extra  starting  windings  or  conducting  bridges  are  provided 
between  the  pole-pieces  to  assist  in  starting.  Possibly  the 
manufacturer  in  shipping  may  have  omitted  these  devices. 
In  such  cases  one  must  refer  to  the  factory. 

Usually,  as  above  suggested,  compensators  are  used  for  start- 
ing synchronous  motors.  If  there  is  a  reversed  phase  in  a 
compensator,  or,  if  the  windings  of  the  armature  of  the  syn- 
chronous motor  are  connected  incorrectly,  there  will  be  little 
starting  torque.  Incorrect  connection  can  be  located  by  not- 
ing the  unbalanced  entering  currents.  Readings  to  determine 
this  unbalancing  should  be  taken  with  the  armature  revolv- 
ing slowly.  The  revolving  can  be  effected  by  any  mechanical 
means.  While  the  motor  is  standing  still,  even  with  correct 
connections,  the  armature  currents  of  the  three  phases  usually 
differ  somewhat.  This  is  due  to  the  position  of  the  poles  in 


SEC.  9]    SYNCHRONOUS  MOTORS  AND  CONDENSERS          237 

relation  to  the  armature,  but  when  revolving  slowly,  the  cur- 
rents should  average  up.  If  the  rotor  cannot  be  revolved 
mechanically,  similar  points  on  each  phase  of  the  armature 
must  be  found.  Then,  when  the  rotor  is  set  successively  at 
these  points,  the  currents  at  each  setting  should  be  the  same. 
Each  phase  when  located  in  a  certain  specific  position  as  related 
to  a  pole,  should,  with  right  connections,  take  a  certain  specific 
current.  With  wrong  connections,  the  currents  will  not  be 
the  same. 

358.  Open  Circuit  in  the  Field  of  a  Synchronous  Motor. — 
If  in  the  operation  of  a  synchronous  motor  the  field  current 
breaks  for  any  reason,  the  armature  current  will  largely  in- 
crease, causing  either  a  shutdown  or  excessive  heat.     It  be- 
comes important,  therefore,  in  synchronous  motors  to  have  the 
field  circuit  permanently  established. 

359.  A  Short-circuit  in  an  Armature  Coil  of  a  Synchronous 
Motor  burns  it  out  completely,  charring  it  down  to  the  bare 
copper.     When  this  occurs,  the  symptoms  are  so  evident  that 
there  is  no  difficul  y  in  identifying  the  trouble.     Such  a  coil 
may  under  ordinary  circumstances  be  cut  out  and  operation 
continued.     In  an  induction  motor,  the  current  in  the  short- 
circuited  coil  rises  only  to  a  certain  value,  but  heats  it  many 
times  more  than  normal.     It  is  not  necessarily  burned  out 
immediately,  and  perhaps  it  may  not  be  burned  out  at  all. 

360.  Hunting  of  Synchronous  Motors. — Synchronous  mo- 
tors, served  by  certain   primary  sources   of  energy,  tend  to 
"hunt."     The  periodicity  of  the  swinging  is  determined  by 
properties  of  the  armature  and  the  circuit.     It  may  reach  a 
certain  magnitude  and  there  stick,  or  the  swinging  may  in- 
crease until  finally  the  motor  breaks  down  altogether.     This 
trouble  usually  occurs  on  long  lines  having  considerable  re- 
sistance between  the  source  of  energy  and  the  synchronous 
motor.     Sometimes  it  occurs  under  the  most  favorable  condi- 
tions.    Irregular  rotation  of  a  prime  mover  (Art.  289),  such 
as  a  single-cylinder  steam  engine,  is  often  responsible  for  the 
trouble.     The  usual  remedy  is  to  apply  to  the  poles,  bridges 
(Fig.  240)  of  copper  or  brass  in  which  currents  are  induced 
by  the  wavering  of  the  armature.     These  currents  tend  to 


238  ELECTRICAL  MACHINERY  [ART.  361 

stop  the  motion.  Different  companies  use  different  forms  of 
bridges.  When  hunting  or  pulsating  occurs,  and  the  motor 
is  not  already  equipped  wiht  bridges,  it  is  best  to  consult  the 
manufacturer.  In  general,  the  weaker  the  field  on  a  synchro- 
nous motor,  the  less  the  pulsation.  Sometimes  pulsation  may 
be  so  reduced  that  no  trouble  results  by  simply  running  with 
a  somewhat  weaker  field  current. 

361.  Improper    Armature    Connections    in    Synchronous 
Motors. — This  trouble  usually  manifests  itself  by  unbalanced 
entering  currents  and  by  a  negligible  or  very  low  starting 
torque.     The  circuits  should  be  traced  out  and  the  connec- 
tions remade  until  the  three  entering  currents  for  three-phase, 
or  the  two  entering  currents  for  two-phase,  are  approximately 
equal.     These  currents  will  not  be  equal  even  with  correct 
connection  when  the  armature  is  standing  still. 

362.  Polarity  of  Synchronous  Motors. — Since  the  winding 
of  a  synchronous  motor  armature  is  in  series  all  the  way 
around  the  circumference  and  under  all  of  the  poles,  except 
in  exceedingly  rare  cases,  the  trouble  from  a  reversed  pole  is 
much  less  serious  than  with   an  induction  motor  or  direct- 
current  machine.     With  a  reversed  pole  everything  operates 
fairly  well.     The  only  trouble  is  that  the  fields  require  more 
current  than  they  should  because  of  the  pole  that  is  opposing 
the  field.     If,  therefore,  excessive  field  current  is  required  for 
minimum  input  to  a  motor,  it  is  a  good  plan  to  test  the  polar- 
ity of  all  the  spools  with  a  compass. 

363.  Bearing  Troubles  of  Synchronous  Motors  are  similar 
to  those  of  induction  motors  (Art.  402) .     A  difference  is  that, 
with  a  synchronous  motor,  the  air  gap  between  the  revolving 
element  and  the  poles  is  relatively  large,  so  that  the  wearing 
of  the  bearing,  which  throws  the  armature  out  of  center,  is 
not  so  serious  as  with  an  induction  motor.     End  play  should 
be  treated  the  same  as  with  an  induction  motor  (Art.  416). 


SECTION  10 

MANAGEMENT   OF,   AND   STARTING   AND   CON- 
TROLLING DEVICES  FOR  ALTERNATING- 
CURRENT  MOTORS 

364.  The  National  Electrical  Code  Requirements  Relating 
to  the  Installation  of  Alternating-current-motor  Control  Equip- 
ment are  essentially  the  same  as  those  governing  the  installa- 
tion of  direct-current  control  equipment  which  are  discussed 
briefly  in  Art.  120.  For  further  information  in  regard  to 
wiring  requirements  for  motors,  see  author's  WIKING  FOR 
LIGHT  AND  POWER. 


4.- -Circuit 

Breaker 


Three -Phase 

Motor 


Auto    Trans  formers - 

FIG.  241. — Connections  for  a  circuit  breaker  protecting  a  three-phase 

motor. 

365.  Connecting  a  Circuit-breaker  for  Polyphase  Induction 
Motor  Protection.* — Terminals  A  and  B  are  connected  as 
shown  in  Fig.  241  for  three- wire  systems,  two-  or  three-phase. 
The  circuit-breaker  should  be  so  located  in  the  circuit  that  the 
no- voltage  coils  will  be  subjected  to  the  full  voltage  of  the  cir- 
cuit, irrespective  of  the  position  of  the  starting  switch.  Where 
it  is  desired  to  have  the  overload  of  the  circuit-breaker  in- 

*  The  Cutter  Co. 

239 


240  ELECTRICAL  MACHINERY  [ART.  366 

operative  with  the  auto-starter  switch  in  starting  position,  the 
connections  G  and  H  within  the  starter  should  be  removed  and 
the  special  connections  E  and  F  made  instead. 

366.  The  Methods  of  Starting  Induction  Motors  may  be 
listed  as  follows: 

1.  By  Connecting  Directly  to  the  Line. — This  method  is  ordi- 
narily used  only  for  small  motors — those  of  less  than  10  h.p. 
output — because  on  starting  the  motor  takes  an  excessive  cur- 
rent and  the  voltage  regulation  will  be  disturbed  unless  there 
is  ample  generating  capacity  and  the  conductors  are  of  a  gen- 
erous cross-section. 

2.  By  Inserting  Internal  Resistance  in  the  Rotor  Circuit. — 
This  method  is  used  only  with  wound-rotor  machines.     The 
resistance  is  cut  in  or  out  of  the  circuit  by  the  operation  of 
a  switch  on  the  motor  shaft  so  arranged  that  the  handle  of 
the  switch  is  stationary  when  the  rotor  is  turning. 

3.  By  Introducing  External  Resistance  in  the  Rotor  Circuit. — 
This  method  can  be  used  only  with  a  wound-rotor  machine 
having  collector  rings  upon  which  brushes  bear  that  connect 
with  the  resistance.     The  resistance  is  cut  in  or  out  of  the 
rotor  circuit  by  a  controller  somewhat  similar  to  the  ordinary 
direct-current  motor  controller. 

4.  By  Using  a  Transformer  having  Low-voltage  Taps. — A  low 
voltage  can  be  impressed  on  the  motor  at  starting  by  connect- 
ing it  with  a  suitable  switch  to  the  low-voltage  taps. 

5.  With  a  Starting  Compensator  or  Auto-transformer. — This 
is  the  usual  method  for  motors  of  ordinary  capacity  and  is 
similar  to  the  transformer  method  in  that  low  voltage  from 
the  compensator  taps  are  impressed  on  the  motor  at  starting. 

6.  By  Connecting  the  Armature  Coils  in  Star  for  Starting  and 
in  Delta  for  Running. — This  method  is  described  in  detail  in 
following  Art.  383. 

367.  A  Small  Induction  Motor  can  be  Started  by  Throwing 
it  Directly  on  the  Line  (Fig.  242). — This  method  is,  as  a  gen- 
eral thing,  not  used  for  motors  of  capacities  exceeding  5  h.p. 
Two  sets  of  fuses  should  be  provided,  one  for  starting  and  one 
for  running,  with  a  double-throw  switch  to  connect  the  motor 
to  either  set.     A  switch,  having  a  spring  so  arranged  that  the 


SEC.  10] 


ALTERNATING-CURRENT  MOTORS 


241 


blades  will  not  remain  in  the  starting  position  unless  manually 
held  there,  should  be  used.     The  starting  current  of  an  in- 


.  Starting  Fuses 


Motor 


FIG.  242. — Starting  small  motor  by  throwing  directly  on  the  line. 

TbLlne 


Fit) ft  orCinui'tBrfaktn.\ 


Wound  Rotor 
Alternating  Current. 


Secondary  Leads 


FIG.  243. — Connections  of  starter  to  wound-rotor  motor. 

duction  motor  thrown  directly  on  the  line  will  be  something 
between  3  and  8  times  the  full-load  running  current.     If  only 
one  set  of  fuses  is  used  for  a  polyphase 
motor  and  they  are  of  sufficient  capac- 
ity to  carry  the  starting  current,  one 
fuse  may  open  but  the  motor  will  con-     .  . 


closing 
tinue  to  operate  on  one  phase,  drawing  Switching 

a  current  considerably  above  normal. 
The  probable  result  is  a  burnt-out 
motor. 

368.  Self-contained  Starters  for 
Wound-rotor  Induction  Motors  of  Rel- 
atively Small  Capacity  (Figs.  243  and 
244)  can  be  purchased.  The  resistors 

for  these  are  mounted  within  the  enclosing  case  that  carries  the 
switching  mechanism  that  increases  or  decreases  the  amount 

16 


FIG.  244.  —  Enclosed 
starter  for  a  phase-wound 
motor. 


242 


ELECTRICAL  MACHINERY 


[ART.  369 


of  effective  resistance  in  the  rotor  circuit.  As  a  rule,  the  re- 
sistors in  these  starters  are  designed  only  for  starting  service, 
hence  they  can  be  used  only  where  starts  are  infrequent  and 
starting  conditions  are  not  severe.  They  are  not  usually  de- 
signed for  speed  control  for  which  service  drum-type  controllers 
with  externally  mounted  resistances  are  used.  In  the  usual 
designs  a  set  of  resistors  is  connected  in  series  with  each  phase 
of  the  motor  (Fig.  245)  secondary  and  all  three  are  intercon- 
nected in  star  by  the  frame  of  the  starter  which  is  grounded, 
protecting  the  operator  against  shocks. 

369.  In  Operating  a  Self-contained  Starter  for  a  Wound- 
rotor  Motor  (Figs.  243  and  244)  before  closing  the  primary 
line  switch  or  breaker,  the  handle  of  the  starter  must  be  in 
the  starting  position,  where  all  the  starting  resistance  is  in 


Adjustable 
/    Arm 


FIG.  245. — Method  of  varying  rotor  resistance  of  a  wound-rotor 
induction  motor. 

circuit.  If  the  connections  are  correct,  and  the  load  is  not 
too  great,  the  motor  should  start  as  soon  as  the  line  switch 
is  closed;  on  failure  to  start,  open  the  primary  circuit,  and  ex- 
amine the  load  conditions  and  the  connections.  With  some 
starters  the  handle  may  have  to  be  advanced  slightly  beyond 
the  starting  position  before  the  motor  starts.  As  the  motor 
speed  accelerates  the  starter  handle  should  be  moved  gradu- 
ally to  the  running  position,  bringing  the  motor  to  full  speed 
within  the  time  which  is  usually  specified  by  the  manufacturer 
of  the  starter.  In  the  running  position  all  starting  resistance 
is,  in  starters  of  most  designs,  short-circuited. 

370.  Starting  a  Coil-wound  Rotor  Motor.* — With  the  coil- 
wound  rotor,  high  and  variable  starting  torque  can  be  obtained 
by  inserting  a  variable  ohmic  resistance  directly  in  the  rotor 

*  SOUTHERN  ELECTRICIAN.   • 


SEC.   10] 


ALTERNATING-CURRENT  MOTORS 


243 


circuit.  The  rotor  circuit  is  connected  to  a  non-inductive  re- 
sistance, which  can  be  varied  and  gradually  cut  out  as  the 
motor  attains  speed.  Figs.  245  and  246  illustrate  the  con- 
nections. When  the  rheostat  handle  is  in  the  extreme  left-hand 
position,  the  resistance  is  all  out  of  circuit.  To  start  the  motor, 
current  is  first  switched  on  to  the  stator  circuit  by  closing  a 
triple-pole  switch.  The  three-pole  contact  blades  of  the  start- 
ing rheostat  are  now  moved  over  from  the  off  position  on  to 
the  resistance  studs,  the  first  contacts  of  which  place  the  whole 
of  the  resistance  in  circuit  with  the  respective  three-phase 
windings  of  the  rotor.  This  prevents  the  current  induced  in 
the  rotor  windings  by  the  stator  circuit  from  reaching  an  ex- 
cessive intensity.  The  switch  handle  on  being  further  rotated 


Three  -  Phase  Mains 


FIG.  246. — Starting  arrangement  for  three-phase  coil-wound  rotor  motor. 

in  a  right-hand  direction  gradually  cuts  out  the  resistance 
until  all  the  resistance  is  out  of  circuit.  In  this  position  the 
rotor  windings  are  short-circuited. 

371.  Commercial  Starting  Compensators  for  Squirrel-cage 
Induction  Motors  usually  have  three  positions  at  which  the 
starting  lever  will  come  to  rest — an  "off"  position,  a  "start- 
ing" position,  and  a  "running"  position.  The  lever  is  so  ar- 
ranged that  the  switch  which  it  controls  cannot  come  to  rest 
in  any  other  positions  unless  forcibly  restrained.  The  con- 
nections of  a  two-phase  and  of  a  three-phase  compensator 
are  shown  in  connection  with  the  material  on  auto-trans- 
formers in  Sec.  Vof  the  author's  AMERICAN  ELECTRICIANS' 
HANDBOOK.  Connection  arrangements  for  compensators  of 
other  types  are  shown  on  pages  adjacent  hereto. 


244  ELECTRICAL  MACHINERY  [ART.  372 

In  starting  compensators,  as  usually  arranged,  when  in  the 
"off"  position  the  switch  is  open  and  the  motor  and  auto- 
transformer  a're  entirely  disconnected  from  the  source  of  en- 
ergy. When  in  the  " starting"  position,  the  source  of  energy 
is  directly  connected  by  the  switch  to  the  auto-transformer 
terminals  and  the  low-voltage  taps  of  the  auto-transformer 
are  connected  to  the  motor.  Usually  there  are  no  fuses  in- 
serted in  the  starting  leads  at  the  compensator. 

When  thrown  to  the  " running"  position  the  switch  connects 
the  motor  through  fuses  to  the  source  of  energy  and  the 
auto-transformer  is  entirely  disconnected  from  the  source  of 
energy.  The  fuses  provided  in  the  running  leads  are  for  the 
protection  of  the  mo.tor  against  overload  while  it  is  in  normal 
operation.  The  fuses  protecting  the  tap  circuit  to  the  com- 
pensator where  the  tap  circuit  branches  from  the  main  are 

2|  Amp. 


100  Volte  He 

^  2{lbs.Torqu* 


.fWfthout  Compensator.  IE. With  Compensator. 

FIG.  247. — Starting  with  and  without  compensator. 

usually  depended    upon   to    protect  the  motor   while    it   is 
starting. 

372.  Starting  With  and  Without  Compensators.— The  start- 
ing current  taken  by  a  squirrel-cage  induction  motor  at  the 
instant  of  starting  is  equal  to  the  applied  electromotive  force 
divided  by  the  impedance  of  the  motor.  Only  the  duration  of 
this  current,  and  not  its  value,  is  affected  by  the  torque  against 
which  the  motor  is  required  to  start.  The  effect  of  starting 
without  and  with  a  compensator  is  illustrated  by  diagrams  / 
and  II  in  Fig.  247.  In  this  diagram,  motor  /  is  thrown  directly 
on  a  100-volt  line.  The  impedance  of  the  motor  is  5.77  ohms 
per  phase,  the  starting  torque  10  Ib.  at  1-ft.  radius  and  the 
current  taken  10  amp.  In  diagram  II  a  compensator  is  in- 
serted, stepping  down  the  line  pressure  from  100  to  50  volts. 
This  reduces  the  starting  current  of  motor  one-half  and  the 


SEC.  10] 


ALTERNATING-CURRENT  MOTORS 


245 


starting  torque  becomes  one-quarter  its  previous  value  or 
Ib.  at  1-ft.  radius.  The  current  in  the  line  is  reduced  inversely 
as  the  ratio  of  transformation  in  the  compensator  and 
becomes  2J^  amp. 

373.  When  a  Compensator  is  Used  the  Starting  Torque 
of  the  Motor  can  be  Reduced  to  Approximately  the  Value 
Required  by  the  Load  and  the  current  taken  from  the  line 
correspondingly  decreased.  Where  a  compensator  is  not  used, 
an  increase  of  rotor  resistance  results  in  a  proportional  in- 
crease in  the  starting  torque  of  the  motor  with  a  very  slight 
decrease  in  the  starting  current  drawn  from  the  line.  Where 
a  compensator  is  used  with  a  motor  having  a  high-resistance 
rotor  the  voltage  can  be  reduced  to  a  lower  value  than  would 


1 00  Amp 


BOAmp.      lOOAmp. 


I.  Starting  with 
Auto  -Transformer 


H  Running  Connection^ 


]tt.  Starting  with 
Resistance. 


FIG.  248. — Starting  with  resistance  and  with  compensator. 

be  required  with  a  low-resistance  rotor  for  the  same  starting 
torque.  Standard  compensators  are  provided  with  several 
taps  from  which  various  combinations  can  be  obtained. 

374.  Comparison  of  Auto-transformer  and  Resistance  for 
Decreasing  Voltage  for  Starting  Squirrel-cage  Motors. — The 
motor  in  Fig.  248  is  supposed  to  require  100  amp.  to  start 
it;  that  is,  to  provide  the  energy,  which  will  produce  the 
necessary  starting  torque.  At  7,  where  an  auto-transformer 
is  used  to  lower  the  voltage  to  110,  a  current  of  100  amp. 
is  produced  in  the  motor  primary  with  a  current  in  the  line 
of  50  amp.  This  condition  is  due  to  the  transformer  action 
of  the  auto-transformer.  At  //  the  running  connections  are 
shown  wherein  the  autotransformer  is  entirely  disconnected 
from  the  circuit.  At  III  are  illustrated  the  conditions  that 
would  obtain  were  the  voltage  lowered  for  starting  by  insert- 


246 


ELECTRICAL  MACHINERY 


[ART.  375 


ing  resistance  in  series  with  the  line.  Obviously  100  amp. 
must  flow  in  all  portions  of  the  line  even  though  the  resistance 
of  1.1  ohms  reduces  the  line  voltage  of  220  to  a  voltage  of  110 
which  is  impressed  on  the  motor.  There  is  a  loss  of  energy 
(watts)  in  the  resistance.  Evidently  the  auto-starter  method 
is  preferable  because  with  it  the  line  current  is  reduced  and 
there  is  practically  no  loss  of  energy.  Although  the  example 
illustrated  is  for  a  two-phase  motor  the  principle  is  the  same 
for  a  three-phase  motor. 

375.  Approximate  Starting  Currents  and  Starting  Torques 
of  Squirrel-cage  Induction  Motors  with  Different  Impressed 
Voltages  Obtained  by  Using  a  Compensator  Starter. — Starting 
current  and  starting  torque  are  expressed  in  terms  of  normal 
full-load  current  and  full-load  torque,  and  impressed  voltage 
is  expressed  in  terms  of  normal  voltage: 


Voltage   impressed   on 
motor,  per  cent. 

Starting  current  taken 
from  line,  per  cent. 

Starting  torque, 
per  cent. 

40 

112 

32 

60 

250 

72 

80 

450 

128 

100 

700 

200 

376.  Taps  of  a  Starting  Compensator.* — Compensators  are 
usually  shipped  by  their  manufacturers  connected  to  the  auto- 
transformer  tap  giving  the  lowest  torque.  If  the  motor  will 
not  start  its  load  with  this  tap  connected  the  next  higher 
voltage  tap  should  be  tried,  and  so  on,  until  the  tap  is  found 
that  provides  the  required  torque.  Compensators  for  use  with 
motors  of  15  h.p.  and  under  sometimes  have  three  taps  giving 
voltages  of  40  per  cent.,  60  per  cent,  and  80  per  cent,  of  full- 
line  impressed  voltage.  For  motors  above  15  h,p.,  four  taps 
are  frequently  provided  giving  40,  58,  70  and  85  per  cent, 
of  full-line  voltage.  The  proper  tap  for  giving  the  maximum 
starting  torque  without  causing  an  inconvenient  voltage  dis- 
turbance in  the  supply  circuit,  can  best  be  ascertained  by 
experiment. 

*  SOUTHERN  ELECTRICIAN. 


SEC.  10] 


ALTERNATING-CURRENT  MOTORS 


247 


One  make  of  compensator  has  for  motors  of  from  5  to  18 
h.p.,  taps  starting  the  motor  at  50,  65  and  80  per  cent,  of  the 
full  impressed  line  voltage,  with  respective  line  currents  equal 
to  25,  42  and  65  per  cent,  of  the  current  that  would  be  taken 
by  the  motor  if  no  compensator  were  used.  For  motors  larger 
than  18  h.p.,  compensator- voltage  taps  are  provided  giving 
voltages  equal  to  40,  58,  70  and  85  per  cent,  of  the  full  im- 
pressed line  voltage,  and  respective  currents  approximately 
equal  to  16,  34,  50  and  72  per  cent,  of  the  current  that  would 
be  taken  by  the  motor  if  it  were  started  directly  from  the 
supply  line. 


Y-  Connection 


FIG.  249. — Starting  compensator  with  separate  switches,  and  auto- 
transformer  for  high-voltage  or  large  capacity  motor. 

377.  Starting  Compensators  for  Motors  of  High-voltage  or 
Large  Current  Capacity  are  arranged  with  the  switches  sep- 
arate from  the  auto-transformer  (Fig.  249).  The  equipment 
usually  consists  of  one  double-throw  or  two  interlocked  single- 
throw  oil  switches  for  the  motor  and  a  single-throw  oil  switch 
for  energizing  the  auto-transformer.  In  the  running  leads  to 
the  motor  may  be  inserted  overload  relays  which  will  open  the 
oil  switches  in  the  case  of  over-draught  of  current.  The  oil 
switches  are  usually  mounted  on  a  switchboard  panel  while 
the  auto-transformer  may  or  may  not  be  mounted  on  the  panel. 
The  construction  indicated  in  the  other  compensator  diagrams 
is  used  by  certain  manufacturers  for  motors  of  capacities  up 
to  and  including  550  volts  when  the  normal  current  does  not 


248 


ELECTRICAL  MACHINERY 


[ART.  378 


exceed  300  amp.  per  phase  and  for  motors  of  from  1,040  to 
2,500  volts  with  currents  not  greater  than  125  amp.  per  phase. 
Where  motors  take  greater  normal  currents  or  are  of  higher 
voltage  the  arrangement  of  Fig.  249  is  applied. 

378.  When  No-voltage  Release  Compensator  Starters  are 
Used  for  High-voltage  Motors  a  small  voltage  transformer  is 


•load  Relay  s 

*  Potential  Transformer 


Three-Phrte 
Motor 


Running  Side  if 
Oil  Switch  |[ 
Starting  Side  ^f~f~T~VT'T'! 


Auto  -  Transformer' 


FIG.  250. — Potential  transformer  for  no-voltage  relay  of  high-voltage 

motor. 

usually  arranged  as  in  Fig.  250  to  energize  the  no-voltage  coil. 
This  arrangement  is  used  by  certain  manufacturers  for  com- 
pensators, with  the  no-voltage  release  attachment,  for  voltages 


Phase  A: 
Phase, 


Phase  A..,  -*Ph*seB 
'}  Back  Finger  Block     '"^||  Motor 

\  Cylinder 
beneraior         Starring  Side  jjt|fei]p[%] j  Fron  t  Finger  Block 

Taps  I  ^1  %?^\- Auto -Trans  former 
FIG.  251. — Overload  relays  on  a  two-phase  starting  compensator. 

of  from  1,040  to  2,500.  The  secondary  of  the  transformer 
furnishes  110  volts  for  which  the  no- volt  age  relay  is  wound. 
379.  Overload  Release  Coils  on  Compensators  are  arranged 
Essentially  as  Shown  in  Figs.  251  and  252. — When  there  is 
an  overload  on  either  phase  the  iron  plunger  of  the  overload 
relay  is  drawn  up  which  opens  the  no-voltage  release-coil  cir- 


SEC.  10] 


ALTERNATING-CURRENT  MOTORS 


249 


cuit.     This  de-energizes  the  no-voltage  release  coil  and  the 
compensator  circuit  is  automatically  opened  as  described  in 


Fuses 


:  S  7|  Back  Fin  qer  Block 
1 1  Switch  Cylinder 


Three-  Phase 
Motor. 


•Line 

Swfoh 


Box 

Containing 
Overload 
Relay* — 


Starting  6ide  L&Q-SOiJ  Front  Finger  Block 
Auto -Transformer 
FIG.  252. — Overload  release  coils  on  a  three-phase  starting  compensator. 

the  paragraph  on  the  no- voltage  release.  The  overload  relays 
are  usually  arranged  so  that  they 
can  be  adjusted,  to  operate  at  dif- 
ferent currents,  just  as  a  circuit- 
breaker  can  be  adjusted.  An  in- 
verse-time-element feature  is  us- 
ually incorporated  whereby  the 
relay  will  operate  almost  instantly 
on  very  heavy  overloads  but  will 
not  operate  until  a  certain  interval 
of  time  has  elapsed  (the  length  of 
the  interval  being  approximately 
inversely  proportional  to  the 
amount  of  overload)  on  lesser 
overloads.  It  will  be  noted  from 
the  diagrams  that  fuses  are  not 
necessary  where  the  overload  re- 
lays are  used.  A  decided  advan- 
tage of  the  overload  relays  is  that 
they  can  be  adjusted  to  protect  a 
motor  against  running  single- 
phase.  If  one  phase  opens,  sum-  voltage  and 
,  ,.,.  ~  '  .  ,  attachments, 

cient   additional   current  will   be 

drawn  through  the  others  to  operate  a  relay  which  will  open 
the  circuit  to  the  compensator.     An  installation  of  a  Westing- 


Qperating 

fondle-- 


FIG.  253. — Installation  of  an 
auto-starter  equipped  with  no- 
overload    release 


250 


ELECTRICAL  MACHINERY 


[ART.  380 


house  compensator  having  no- voltage  and  overload  relays  is 
shown  in  Fig.  253. 

380.  A  No -voltage  Release  can  be  Provided  on  Starting 
Compensators. — The  connection  diagram  is  shown  in  Fig.  254 
for  a  three-phase  compensator  and  that  for  a  two-phase  com- 
pensator is  similar.  When  a  condition  of  no-voltage  exists  on 


[  -  Overload  Relays 


Generator 


No  Voltaae 
Release  Coil    [ : 

Running  Side  ,- 


;  Back  Finger  Block 
\5witch  Cylinder 
\  Front  Finger  Block 


.Starting  <5ide   si; 

Torps^  ^  ^^-Auto -Transformer 
FIG.  254. — Starting  compensator  with  no-voltage  release. 
Three-Phase  Main  (Normal  Voltage) 


Compensator   Mam  (Reduced  Voltage) 


FIG.  255. — Starting  several  motors  from  one  compensator. 

the  line,  the  no-voltage  release  coil  is  de-energized  which  per- 
mits the  iron  armature  or  core  of  the  no- volt  age  coil  to  drop, 
automatically  releasing  the  compensator  handle,  which  is  re- 
turned to  the  off  position  by  its  spring.  This  opens  the  cir- 
cuit through  the  compensator. 

381.  A  Method  of  Starting  Several  Polyphase  Induction 
Motors  from  One  Compensator  is  shown  in  Fig.  255.     This 


SEC.  10] 


ALTERNATING-CURRENT  MOTORS 


251 


can  frequently  be  employed  to  advantage  where  there  are  a 
number  of  motors  situated  close  together  or  where  a  number 
of  motors  must  be  started  from  one  location.  A  double- 
throw  switch  is  necessary  for  each  motor  to  be  started,  and 
there  should  be  a  switch  for  the  compensator.  If  all  of  the 
starting  switches  are  located  close  together,  so  that  one 
operator  can  open  or  close  them  consecutively,  the  compen- 
sator need  have  a  capacity  only  sufficient  for  serving  the 
largest  motor  in  the  group.  If  the  starting  switches  are  so 
located  that  several  can  be  operated  at  once  by  different 
men,  the  compensator  must  have  a  sufficient  margin  of  ca- 
pacity to  provide  for  this.  After  all  of  the  motors  are  started 
the  compensator  switch  is  opened,  eliminating  compensator 


Three-Phase 
Generator 


Runninq 
Position-^ 


Motor 


Runninq 
Connections. 
(  Defta) 


FIG.  256.  —  The  delta-star  method  of  starting. 


Starting 

Connections. 

(Star). 


losses.     Where  motors  exceed  possibly  7  h.p.  in  capacity,  oil 
switches  should  be  used  for  the  starting  switches. 

382.  Fuses    for    Use    in    Connection   with   Compensator 
Starters.  —  National  Code  standard  fuses  carried  in  holders 
mounted  on  slate  bases  are  usually  used  for  compensators  for 
voltages  up  to  600  volts.     For  voltages  of  from  1,040  to  2,500, 
if  fuses  are  used,  the  expulsion  type  is  preferable.     A  table 
of  fuse  sizes  for  induction  motors  is  given  in  the  author's 
AMERICAN  ELECTRICIANS'  HANDBOOK,  but  where  not  other- 
wise specified,  fuses  of  a  capacity  corresponding  to  1^  times 
the  full-load  current  of  the  motor  are  supplied. 

383.  The    Delta-star    Method    of    Starting  Three-phase, 
Squirrel-cage  Induction  Motors  is  sometimes  used  (Fig.  256). 
The  stator-coil  terminals  are  brought  out  from  the  frame  and 
connected  to  a  double-throw  switch  as  shown.     In  starting, 


252  ELECTRICAL  MACHINERY  [ART.  384 

the  coils  are  connected  in  star  and  the  current  is  1  4-  1.73  or 
0.58  of  what  it  would  be  with  the  coils  connected  in  delta. 
After  the  rotor  has  attained  full  speed  the  switch  is  thrown 
to  the  running  position,  which  connects  the  coils  in  delta  and 
normal  voltage  is  thereby  impressed  on  them.  Motors  must 
be  specially  constructed  for  this  method  of  starting  as  it  is 
not  extensively  used  by  the  principal  manufacturers. 

384.  Speed  Control  of  Polyphase  Motors.* — The  speed  of 
polyphase  induction  motors  can  be  controlled  by  a  number  of 
different  methods,  of  which  the  following  are  the  most  impor- 
tant.    I.  Adjusting  the  resistance  of  the  secondary  circuit. 
II.   Adjusting  the  primary  voltage.     III.  Using  two  motor 
primaries,  one  of  which  is  capable  of  being  rotated.     IV. 
Changing  the  number  of  motor  poles.     V.  Operating  two  or 
more  motors  connected  in  cascade.     VI.   Adjusting  the  fre- 
quency of  the  primary  current.     VII.  Changing  the  number 
of  phases  of  the  secondary  windings. 

The  results  obtained  by  the  use  of  these  various  methods 
differ  widely,  so  that  in  selecting  a  variable-speed  alternating- 
current  motor  careful  consideration  must  be  given  to  the 
characteristics  of  the  method  of  control  in  order  to  determine 
its  suitability  for  the  service.  In  many  cases  a  combination 
of  methods  is  required  in  order  to  produce  the  desired  speed 
changes. 

385.  Speed  Control  of  a  Polyphase  Motor  by  Adjusting 
the  Resistance  of  the   Secondary  Circuit. — With   constant 
torque,  the  speed  of  the  motor  increases  regularly  as  each  step 
of  the  resistor  is  short-circuited  and  remains  constant  on  any 
given  notch.     But  with  varying  torque  the  motor  speed  varies 
also;  that  is,  an  alternating-current  motor  when  operating 
with  auxiliary  resistance  in  the  rotor  circuit  is  properly  classi- 
fied as  a  varying-speed  motor.    This  method  of  speed  control 
is,  therefore,  not  suitable  for  service  requiring  several  constant 
speeds  with  varying  torque,  such  as  machine-tool  work,  etc. 

Speed  control  by  means  of  adjustable  secondary  resistance 
is,  however,  very  useful  where  constant  speeds  are  not  essen- 
tial, for  example,  in  operating  cranes,  hoists,  elevators,  and 

*  B.  G.  Lamme. 


SEC.  10]  ALTERNATING-CURRENT  MOTORS  253 

dredges,  and  also  for  service  in  which  the  torque  remains  con- 
stant at  each  speed,  as  in  driving  fans,  blowers,  and  centrifu- 
gal pumps.  In  service  where  reduced  speeds  are  required 
only  occasionally  and  where  small  speed  variation  is  not  ob- 
jectionable, this  method  of  control  can  also  be  used  to  good 
advantage.  On  account  of  energy  loss  in  the  resistors,  the 
efficiency  is  reduced  when  operating  at  reduced  speeds,  this 
reduction  being  greatest  at  the  slowest  speeds.  The  circuits 
are  essentially  the  same  as  for  starting  by  varying  resistance 
in  the  rotor  circuit,  as  shown  in  Figs.  245  and  246. 

386.  With  Secondary  Speed  Control  the  rotor  usually  has 
a  Y-connected  winding  to  which  is  connected,  in  series  in 
each  phase,  an  external  resistance,  Figs.  245  and  246.     By 
moving  the  adjustable  arm  the  amount  of  resistance  in  series 
in  each  phase  can  be  varied  from  a  maximum  to^zero  and 
the  speed  varied  from  the  highest  speed  to  the  lowest  speed. 
This  form  of  control  is  in  general  preferable  to  the  primary- 
control  method  and  is  used  where  a  larger  number  of  speeds 
is  required  and  it  is  not  necessary  for  the  motor  to  run  at 
any  considerable  period  at  reduced  speed. 

387.  Speed-torque  Graphs  of  a  Secondary  Speed-control 
Induction  Motor  (see  Fig.  257). — To  determine  the  speed  of 
such  a  motor  on  any  point  of  the  controller  when  operating 
against  a  given  torque  and  to  find  the  current  taken  at  that 
speed  and  torque,  refer   to  graphs  which  show  the  speed, 
torque  and  current  for  phase-wound  variable-speed  motors. 
Those  of  Fig.  257  are  typical  of  ordinary  capacities.     For 
any  given  torque,  follow  along  the  abscissa  corresponding  to 
this  value  to  its  point  of  intersection  with  the  torque  curve 
for  that  particular  notch  of  controller.     Then  follow  up  the 
ordinate  until  it  intersects  the  current  curve  corresponding 
to  the  same  controller  notch  and  the  value  so  obtained  is  the 
current  taken  by  the  motor. 

EXAMPLE. — Suppose  it  is  desired  to  determine  the  current  taken  on 
the  various  points  of  the  controller  when  starting  a  25-h.p.  220-volt 
motor  and  bringing  it  from  rest  to  full  speed  against  full-load  torque 
— the  first  point  (Fig.  257)  at  which  more  than  full-load  torque  can 
be  obtained  is  the  third  notch  and  following  the  line  upward  to  the  cur- 


254 


ELECTRICAL  MACHINERY 


[ART.  388 


rent  curve  we  see  that  the  current  taken  is  150  per  cent,  full-load  cur- 
rent. This  value  drops  until  about  45  per  cent,  synchronous  speed  is 
reached,  when  in  order  to  hold  up  the  torque  it  is  necessary  to  throw  to 
the  fourth  notch. 


Per  cent  Synchronism. 

FIG.  257. — Typical  current,  torque  and  speed  curves  for  an  induction 
motor  with  secondary  speed  control. 

The  current  rises  correspondingly  to  130  per  cent,  full-load,  then 
drops  until  53  per  cent,  synchronous  speed  is  reached.  Then  the  con- 
troller must  be  moved  to  the  fifth  notch,  thence  it  drops  until  65  per 


••Induction  Motor 


Induction  Motor^ 


With  Resistance.  With  a  Compensator. 

FIG.  258. — Methods  of  varying  the  voltage  impressed  on  an  induction 

motor. 

cent,  synchronous  speed  is  reached,  etc.     The  dotted  line  indicates  the 
variation  in  current. 

388.  Speed  Control  of  a  Polyphase  Motor  by  Adjusting  the 
Primary  Voltage  (Fig.  258). — Adjusting  the  primary  voltage 


SEC.  10] 


ALTERNATING-CURRENT  MOTORS 


255 


Latch 


of  a  motor  causes  speed  changes  that  are  similar  to  those 
produced  by  adjusting  the  resistance  of  the  motor  secondary. 
The  voltage  variations  can  be  obtained  by  means  of  adjust- 
able resistors,  auto-transformers,  or  choke  coils  in  series  with 
the  primary.  This  method  has  the  disadvantages  of  poor 
speed  regulation,  low  efficiency,  and  unsatisfactory  control, 
especially  when  the  primary  voltage  is  high;  it  is  not  in  gen- 
eral commercial  use.  Squirrel-cage  induction  motors  are, 
however,  almost  invariably  started  with  reduced  primary 
voltage  obtained  by  means  of  auto-transformers.  Fig.  259 
indicates  the  external  appearance  of  a  variable-resistance 
starter  for  such  service. 

389.  Primary  Speed  Control   (Fig.  258).— Where  a  com- 
pensator is  used,  contactors,  connected  by  conductors  to  the 
stator,  are  arranged  to  slide  over 

the  compensator  taps,  in  a  manner 
similar  to  that  in  which  the  lever 
arm  slides  over  the  segments  of  a 
rheostat,  and  thereby  vary  the  volt- 
age impressed  on  the  motor.  The 
speed  regulation  of  a  motor  con- 
trolled by  this  method  is  very  poor 
and  the  power-factor  and  efficiency 
decrease  with  the  speed.  Where  a 
resistance  is  used  for  varying  the  voltage  impressed  on  the 
stator,  the  regulation  and  efficiency  of  the  machine  are  not 
as  good  as  when  a  compensator  is  used. 

390.  Speed  Control  of  a  Polyphase  Motor  with  a  Double 
Primary  Arrangement. — The  double  primary  motor  resembles 
an    ordinary  squirrel-cage  induction  motor  in   construction 
except  that  the  primary  is  divided  vertically  into  halves,  each 
with  separate  core  and  windings.     One-half  can  be  rotated 
around  the  rotor  by  means  of  a  worm-screw  and  rack  device. 
Fig.  260  shows  this  construction.     When  the  two  halves  of 
the  primary  are  placed  so  that  like  poles  are  in  line,  the  rotor 
windings  are  subjected  to  maximum  magnetic  flux  from  the 
primary,  and  the  motor  will  run  with  minimum  slip  and  there- 
fore at  its  maximum  speed.     By  turning  the  movable  half  of 


'Contact 


FIG.  259.  —  Primary  resist- 
ance starter  for  a  squirrel-cage 
motor. 


256 


ELECTRICAL  MACHINERY 


[ART.  391 


the  primary,  the  flux  acting  on  each  rotor  bar  is  gradually 
reduced,  causing  increased  slip  and  a  corresponding  reduction 
of  the  motor  speed  for  a  given  torque. 

This  operation  is  equivalent  to  varying  the  primary  voltage 
and  therefore  cannot  be  used  with  advantage  where  constant 
speed  with  varying  torque  is  desired.  The  mechanism  is, 
however,  self-contained;  the  speed  changes  are  effected  with- 
out opening  circuits;  and  the  motor,  having  no  brushes,  oper- 
ates without  sparking. 

391.  Speed  Control  of  a  Polyphase  Motor  by  Changing  the 
Number  of  Motor  Poles. — The  synchronous  speed  of  a  poly- 
phase motor  is  inversely  proportional  to  the  number  of  its 
poles.  Thus  on  a  60-cycle  circuit  a  two-pole  induction  motor 


Rotating 
Bar  Winding.  ^      Screw-  -. 


Becrriny 


FIG.  260. — Longitudinal  section  of  a  double  primary  motor. 

has  a  synchronous  speed  of  approximately  3,600  r.p.m.,  a  four- 
pole  motor  1,800  r.p.m.,  an  eight-pole  motor  900  r.p.m.,  etc. 
It  is  therefore  possible  to  alter  the  speed  of  a  motor  by 
changing  the  number  of  its  poles. 

This  can  be  accomplished  by  using  two  or  more  separate 
primary  windings,  each  having  a  different  number  of  poles, 
or  by  using  a  single  winding  which  can  be  connected  so  as  to 
form  different  numbers  of  poles.  In  general  only  two  speeds 
are  possible  without  great  complication,  the  preferable  ratio 
being  1:2.  The  rotor  should  be  of  the  squirrel-cage  type  as 
this  is  adapted  to  any  number  of  poles,  whereas  the  windings 
of  a  wound  rotor  must  be  reconnected  for  the  different  speeds. 

With  very  few  exceptions  these  motors  are  squirrel-cage 


SEC.   10]  ALTERNATING-CURRENT  MOTORS  257 

machines  with  special  stator  windings.  They  are  designed 
to  operate  at  full  and  half  speed,  the  different  speeds  being 
obtained  by  changing  the  connection  of  the  coils  so  as  to  halve 
or  double  the  number  of  poles.  Usually  motors  with  the  lower 
speed  other  than  half  speed  require  more  complicated  con- 
nections and  necessitate  bringing  out  a  large  number  of  leads 
from  the  motor.  The  motors  can  be  designed  for  three  or 
four  speeds,  but  such  will  require  two  distinct  stator  windings. 
Obviously,  these  motors  are  very  special  and  their  use  is  not 
advocated  except  when  absolutely  necessary. 

The  efficiency  is  approximately  the  same  at  each  speed  and 
the  power-factor  which  is  lower  at  full  speed  than  that  of 
the  normal  motor  is  reduced  very  greatly  at  the  lower  speed. 
Also  the  output  is  proportional  to  the  speed,  while  the  per- 
centage slip  remains  approximately  the  same  for  each  speed, 


F/rsfMotor 


FIG.  261. — Two  polyphase  motors  connected  in  cascade. 

and  the  starting  torque  per  ampere  varies  approximately  in- 
versely as  the  speed. 

392.  Speed  Control  of  Polyphase  Motors  by  Operating 
Two  or  More  Motors  Connected  in  Cascade  offers,  under 
some  conditions  of  service,  the  most  convenient  and  econom- 
ical method  of  speed  variation.  In  this  arrangement  all  the 
rotors  are  mounted  on  one  shaft  or  the  several  shafts  are 
rigidly  connected.  The  primary  of  the  first  motor  is  con- 
nected to  the  line,  its  secondary,  which  must  be  of  the 
phase-wound  slip-ring  type,  to  the  primary  of  the  second 
motor  and  so  on.  The  secondary  of  the  last  motor  can  be 
either  of  the  squirrel-cage  or  of  the  phase-wound  type.  In 
practice  more  than  two  motors  are  rarely  used.  The 
arrangement  is  shown  in  Fig.  261. 

Speed  changes  are  obtained  by  varying  the  connections  of 

17 


258  ELECTRICAL  MACHINERY  [ART.  393 

the  motors,  the  following  combinations  being  possible  with 
two  motors:  Each  motor  can  be  operated  separately  at  its 
normal  speed  with  its  primary  connected  to  the  line,  the 
other  motor  running  idle;  the  motors  can  be  connected  in 
cascade  so  that  the  rotors  tend  to  start  in  the  same  direc- 
tion (direct  concatenation);  or  the  motors  can  be  connected 
so  that  the  rotors  tend  to  start  in  opposite  directions 
(differential  concatenation).  If  the  first  motor  has  12  poles 
and  the  second  4,  the  following  synchronous  speeds  can  be 
obtained  on  a  25-cycle  circuit. 

(1)  Motor  II  (4  poles)  running  single,  750  r.p.m.;  (2) 
motors  in  differential  cancatenation  (equivalent  of  8  poles), 
375  r.p.m.;  (3)  motor  I  (12  poles)  running  single,  250  r.p.m.; 
(4)  motors  in  direct  concatenation  (equivalent  of  16  poles), 
187.5  r.p.m.  By  the  use  of  adjustable  resistance  in  the 
secondary  circuits,  changes  from  one  speed  to  the  next  can  be 
made  with  uniform  gradations. 

A  great  number  of  speed  combinations  are  possible  by  the 
use  of  this  method;  the  control  is  simple  and  safe,  as  few 
leads  are  required  and  main  circuits  are  not  opened  for  most 
of  the  speeds.  The  rotors  can  be  made  with  smaller  diame- 
ters than  is  possible  with  other  multispeed  motors,  hence  the 
flywheel  effect  is  reduced  to  a  minimum.  In  general,  a  cascade 
set  is  applicable  where  speed  changes  must  be  frequently 
made  with  high  horse-power  output  and  primary  voltage, 
and  where  the  speed  ratios  are  other  than  1 :2. 

393.  Speed  Control  of  a  Polyphase  Motor  by  Adjusting  the 
Frequency  of  the  Primary  Current. — Since  the  synchronous 
speed  of  an  induction  motor  is  equal  to  the  alternations  of 
the  supply  circuit  divided  by  the  number  of  poles  in  each 
circuit,  a  change  in  speed  can  be  effected  by  changing  the 
frequency  of  the  circuit. 

Fig.  262  shows  the  speed-torque  and  other  curves  of  a  motor 
when  operated  at  7,200,  3,600,  1,800,  and  720  alternations  per 
minute,  or  at  100,  50,  25,  and  10  per  cent,  of  the  normal 
alternations.  The  speed-torque  curves  corresponding  to  the 
above  alternations  are  a,  &,  c,  and  d.  The  current  curves 
are  A,  B,  C,  and  D.  This  figure  shows  that  for  the  rated 


SEC.  10] 


ALTERNATING-CURRENT  MOTORS 


259 


torque  Tt  the  current  is  practically  constant  for  all  speeds, 
but  the  electromotive  force  varies  •  with  the  alternations. 
Consequently,  the  apparent  power  supplied,  represented  by 
the  product  of  the  current  by  electromotive  force,  varies  with 


Torque. 


FIG.  262. — Performance  curves  of  a  polyphase  induction,  motor  with 
different  applied  frequencies  and  different  applied  electromotive  forces. 

the  speed  of  the  motor,  and  is  practically  proportionate  to 
the  power  developed. 

In  a  few  cases,  where  only  one  motor  is  operated,  the 
generator  speed  can  be  varied.  If  the  generator  is  driven  by 
a  waterwheel,  its  speed  can  be  varied  over  a  wide  range,  and 


varied-* 


Induction 


Induction  Motor 


Motor-* 


FIG.  263.  —  Speed  adjustment  by  changing  frequency. 


the  motor  speed  will  also  vary.  If  the  generator  field  is  held 
at  practically  constant  strength,  then  the  motor  speed  can 
be  varied  from  zero  to  a  maximum  at  constant  torque  with 
a  practically  constant  current. 

Another  method  of  accomplishing  this  result  is  by  the  use 
of  a  frequency  changer.      Fig.  263  shows  the  arrangement. 


260 


ELECTRICAL  MACHINERY 


[ART.  393 


B  and  C  are  induction  motors  of  the  ordinary  type;  A  is  a 
direct-current  motor  directly  connected  to  the  rotor  of  B. 
C  is  the  driving  motor  and  B  the  frequency  changer.  The 
primary  of  B  is  connected  to  the  line,  its  secondary  to  the 


Sfdrtor  or  Primary  W/ndirtg 


Lines 


—•Secondary 
Resistance 


^Kofor  or  Secondary  Winding 


FIG.  264. — One  secondary  circuit  closed  (changing  the  number  of  phases 
of  the  secondary  winding). 

primary  of  C.  The  frequency  of  the  current  delivered  to  C 
depends  on  the  relation  of  the  speed  of  the  rotor  B  to  the 
synchronous  speed  of  B',  the  slower  the  rotation  of  the  rotor 


I- for  Single-Phase  Motor  I-  for  Three-Phase  Motor 

FIG.  265. — Connections  for  automatic  starters  for  alternating-current 

motors. 

the  higher  the  frequency  dehvered  to  C  and  the  higher  the 
speed  of  C.  The  speed  of  the  rotor  B  is  controlled  by  adjust- 
ing the  field  of  motor  A.  Motor  B  must  be  practically  the 
same  size  as  G;  but  motor  A  can  generally  be  relatively 


SEC.  10] 


ALTERNATING-CURRENT  MOTORS 


261 


smaller,  the  exact  size  depending  on  the  maximum  and 
minimum  frequency  and  the  power  required  for  motor  C. 
This  method  can  be  applied  with  special  advantage  where 
direct-current  motor  drive  is  not  desirable. 

394.  Speed  Control  of  a  Polyphase  Motor  by  Changing  the 
Number  of  Phases  of  the  Secondary  Winding. — If  only  one 
of  the  secondary  circuits  is  closed  the  motor  will  run  at 
about  half  speed,  with  very  low  power-factor  and  poor 


FIG.  266. — Arrangement  of  float  switch  control  for  a  single-phase  motor. 

efficiency.  This  method  of  speed  adjustment  (Fig.  264)  is 
frequently  used  in  experimental  work,  but  has  no  extensive 
commercial  applications. 

395.  The  Methods  of  Connecting  Float-control  Automatic 
Starters  for  Alternating-current  Pump  Motors  are  illustrated 
by  the  typical  diagrams  of  Fig.  265.  Fig.  266  shows  the  wir- 
ing of  a  non-automatic  float  switch  used  with  a  single-phase 
motor  of  the  type  which  does  not  require  an  automatic  starter. 


262  ELECTRICAL  MACHINERY  [ART.  396 

The  principle  of  operation  of  this  float  switch  is  shown  in  Fig. 
114.  Where  the  pump  motor  is  controlled  by  a  pressure  regu- 
lator, the  connections  for  an  alternating-current  motor  are 
essentially  the  same  as  those  for  a  direct-current  motor  and 
are  illustrated  in  Fig.  115. 

396.  To  Reverse  the  Direction  of  Rotation  of  a  Polyphase 
Induction  Motor. — For  a  two-phase,  four-wire  motor,  inter- 
change the  connections  of  the  two  leads  of  either  phase.  For 
a  two-phase,  three-wire  motor,  interchange  the  two  outside 
leads.  For  a  three-phase  motor,  interchange  the  connections 
of  any  two  motor  leads. 


SECTION  11 

TROUBLES    OF    ALTERNATING-CURRENT     GENERA- 
TORS AND  MOTORS 

397.  The  Troubles  of  Alternating-current  Generators  and 
Motors  are  in  some  respects  similar  to  those  encountered  with 
direct-circuit  machines,  and  the  methods  of  rectifying  them 
are,  in  some  cases,  similar.     However,  since  the  construction 
of  the  alternating-  is  radically  different  from  that  of  the  direct- 
current  apparatus,  it  follows  that  each  type  (a.c.  and  d.c.) 
will  be  subject  to  certain  troubles  peculiar  to  itself.     The  reader 
is  advised  to  review  carefully  Sec.  4,  "Troubles  of  Direct-cur- 
rent Generators  and  Motors"  because  such  portions  of  that 
section  as  may  apply  to  alternating-current  machines  will  not 
be  repeated  in  this  section.     The  methods  there  (Sec.  4)  de- 
scribed of  locating  field-coil  (Art.  223)  and  insulation-resistance 
(245)  troubles  can,  with  obvious  modifications,  be  applied  for 
alternating-current  revolving  fields.     Considerable  of  the  ma- 
terial in  this  section  is  based  on  that  in  the  book,  MOTOR 
TROUBLES  by  E.  B.  Raymond. 

398.  Bearing  Troubles  of  Alternating-current  Machinery 
are  due  to  the  same  causes  that  originate  difficulties  with 
bearings  in  direct-current  units.     Hence,  for  information  on 
this  subject  see  Art.  230  and  following  articles  in  Sec.  4, 
"Troubles  of  Direct-current  Generators  and  Motors."     See 
following  Art.  402  for  discussion  of  induction-motor  bearing 
troubles. 

399.  Troubles   of   Alternating-current   Generators.* — The 
following  causes  may  prevent  alternating-current  generators 
from  developing  their  normal  e.m.f. : 

(1)  The  speed  of  the  generator  may  be  below  normal.     (2) 
The  switchboard  instruments  may  be  incorrect  and  the  voltage 

•  WEBTINQHOUSE  INSTBUCTION  BOOK. 

263 


264  ELECTRICAL  MACHINERY  [ART.  400 

may  be  higher  than  that  indicated,  or  the  current  may  be 
greater  than  is  shown  by  the  readings.  (3)  The  voltage  of 
the  exciter  may  be  low  because  its  speed  is  below  normal,  or 
its  series  field  reversed,  or  part  of  its  shunt  field  reversed  or 
short-circuited.  (4)  The  brushes  of  the  exciter  may  be  in- 
correctly set.  (5)  A  part  of  the  field  rheostat  or  other  un- 
necessary resistance  may  be  in  the  field  circuit.  (6)  The 
power-factor  of  the  load  may  be  abnormally  low. 

400.  Induction     Motor     Troubles.* — The     unsatisfactory 
operation  of  an  induction  motor  may  be  due  to  either  ex- 
ternal or  internal  conditions.     The  voltage  or  the  frequency 
may  be  wrong,  or  there  may  be  an  overload  on  the  machine. 
Low  voltage  is  the  most  frequent  cause  of  trouble.     The  start- 
ing current  is  sometimes  twice  the  running  current,  with  the 
result  that  the  voltage  is  particularly  low  at  starting.     The 
best  remedy  for  this  disorder  is  larger  transformers  and  larger 
motor  leads,   one  or  both.     The  troubles  that   occur  most 
frequently  within  the  motor  itself  are  caused  by  faulty  insu- 
lation, and  by  uneven  air  gap  due  to  the  springing  of  the  motor 
shaft  or  to  excessive  wear  in  the  bearings.     If  a  wound-rotor 
machine  refuses  to  start,  the  trouble  may  be  due  to  an  open 
circuit  in  the  rotor  winding.     A  short-circuited  coil  in  the 
motor  will  make  its  existence  known  by  local  heating  in  the 
latter.     Most  motors  designed  to  employ  a  starting  resistance 
will  not  start  at  all  if  the  resistance  is  omitted  from  the 
secondary  circuit. 

401.  Causes  of  Shutdowns  of  Induction  Motors. — Some- 
times there  is  trouble  from  blowing  fuses.     Or  possibly,  and 
more  serious,  the  fuses  do  not  blow  and  the  motor,  perhaps 
humming  loudly,  comes  to  a  standstill.     Under  these  condi- 
tions, the  current  may  be  10  times  normal,  so  that  the  heat- 
ing effect,  being  increased  as  the  square  of  the  current,  or  100 
fold,  causes  the  machine  to  burn  out  its  insulation.     Since  the 
torque  or  turning  power  of  an  induction  motor  is  proportional 
to  the  square  of  the  applied  voltage  (one-half  voltage  produces 
only   one-quarter  torque),   it   is  evident  that  lowering  the 
voltage  has  a  decided  effect  upon  the  ability  of  the  motor  to 

*  H.  M.  Nichols,  POWER  AND  THE  ENGINEER. 


SEC.  11]     TROUBLES  OF  GENERATORS  AND  MOTORS  265 

carry  load,  and  may  be  the  cause  of  its  stopping,  Another 
cause  may  be  that  the  load  on  the  motor  is  more  than  equal 
to  its  maximum  output. 

402.  Bearing  Troubles  in  Induction  Motors. — The  bearings 
may  have  become  worn,  so  that  the  air  gap  (which  ordinarily 
is  not  much  over  0.040  in.  and  on  small  motors  as  small  as 
0.015  in.)  has  been  gradually  reduced  at  the  lower  side  of  the 
rotor  to  practically  zero.     The  rotor  commences  to  rub  on  the 
stator.     The  friction  soon  becomes  so  great  that  it  is  more  than 
the  motor  can  "pull."     The  result  is  that  it  shuts  down.     A 
shutdown  may  be  due  to  bearings  introducing  excessive  friction. 
Hot  bearings,  in  turn,  may  be  due  to  excess  of  belt  tension, 
dirt  in  the  oil,  oil  rings  not  turning,  or  to  improper  alignment 
of  the  motor  to  the  machine  that  it  drives.     Hence,  under 
such  conditions,  it  should  be  ascertained  whether  the  voltage 
has  been  normal,  whether  the  air-gap  is  such  that  the  rotor 
is  free  from  the  stator,  and  whether  the  load  imposed  upon  the 
motor  is  more  than  that  for  which  it  was  designed.     In  any 
installation  a  system  should  be  arranged  whereby  an  inspector 
will  examine  the  gap,  bearings,  etc.,  periodically.     See  Art.  233 
for  information  relating  to  ball  bearings  for  motors. 

403.  Starling  Switch  Troubles  in  Wound-rotor  Motors. — 
Rarely,  shutting  down  may  be  due  to  the  working  out  of  the 
starting  switch,  which  may  be  located  within  the  armature. 
Such  a  switch  is  operated  by  a  lever  engaging  a  collar  which 
bears  on  contacts  which,  as  they  move  inward,  cut  out  the 
resistance  in  series  with  the  rotor  winding  and  located  within 
it.     If  the  short-circuiting  brushes  work  back,   introducing 
resistance  into  the  armature  circuit  while  the  machine  is  try- 
ing to  carry  load,  it  will  at  once  slow  down  in  speed  and 
probably  stop,  usually  burning  out  the  starting  resistance. 
Of  course,  this  can  occur  only  from  faulty  construction.     The 
remedy  is  to  fit  the  brushes  properly,  so  that  they  will  not 
work  out. 

404.  Low  Torque  while  Starting  Induction  Motors. — Al- 
though the  circuit  to  the  motor  be  closed,  sometimes  it  does 
not  start.     The  same  general  laws  of  voltage,  etc.,  apply  to 
the  motor  at  starting  as  when  running.     Hence,  the  points 


266  ELECTRICAL  MACHINERY  [ART.  405 

mentioned  under  "shutdowns"  (Art.  401)  should  be  investi- 
gated and  if  necessary  corrected.  The  resistance,  which  is 
frequently  inserted  in  the  armature,  may  be  short-circuited, 
thus  giving  a  low  starting  torque.  Unless  a  starting  com- 
pensator is  used  for  starting,  it  is  necessary,  in  order  to 
obtain  a  proper  starting  torque  with  a  reasonable  current,  that 
a  resistance  be  inserted  in  the  rotor  circuit.  The  resistance 
not  only  limits  the  current,  which  would,  with  the  motor 
standing  still,  be  large,  but-  it  causes  the  current  of  the  arma- 
ture to  assume  a  more  effective  phase  relation,  so  that  with 
the  same  current  a  far  larger  torque  is  obtained.  A  partial 
or  complete  short-circuit  of  the  resistance  partially  or  wholly 
ruins  the  starting  torque. 

405.  Low  Maximum  Output  of  Induction  Motors. — The 
maximum  load  which  a  motor  can  carry  may  be  less  than 
desired,  or  less  than  the  nameplate  indicates.  If  the  vol- 
tage, air  gap,  load,  etc.,  are  right,  it  may  be  possible  that  a 
mistake  has  been  made  in  connections.  It  is  then  easiest  to 
return  the  motor  to  the  factory,  but  if  immediate  operation 
is  essential,  the  armature  connections  can  readily  be  changed 
so  as  to  give  a  large  increase  in  output.  To  ascertain  what  to 
do,  remove  the  bracket  on  the  side  of  the  motor  which  covers^ 
the  connections  between  the  coils.  Pick  out  one  phase,  and 
find  out  how  many  groups  of  coils  are  connected  up.  From 
this,  the  number  of  poles  can  be  determined.  A  better  way 
is  to  calculate  this  from  the  speed  of  the  motor  and  the  fre- 
quency of  the  circuit  on  which  it  is  running.  See  315. 

From  an  examination  of  the  connections  it  can  be  easily 
determined  whether  the  poles  in  any  place  are  connected  in 
series  or  in  multiple,  or  in  series-multiple.  Thus,  in  a  motor 
the  connections  may  be  as  shown  at  the  left  in  Fig.  267, 
which  indicates  the  windings  of  one  phase  of  a  four-pole  motor 
If  the  connections  be  changed  to  those  shown  at  the  right 
in  Fig.  267,  each  coil  will  then  receive  double  its  former  vol- 
tage and  the  motor  will  give  4  times  the  output.  Before  mak- 
ing a  change  in  connections  such  as  that  indicated  here  one 
must  ascertain  to  a  certainty  that  the  increased  current  that 
will  result  will  not  injure  the  windings. 


SEC.  11]    TROUBLES  OF  GENERATORS  AND  MOTORS 


267 


It  should  be  borne  in  mind,  however,  that  this  renders 
the  motor  less  efficient,  increasing  the  exciting  current,  and 
thus  lowering  the  power-factor.  If  conditions  demand  it, 
this  method  may  be  followed.  The  temperature  under  the 
new  conditions  should  be  carefully  observed  to  insure  that 
there  is  no  undue  heating.  The  only  change  in  connections 


Coils  Connected  in  Series.  Coils  Connected  in  Series  Parallel. 

FIG.  267. — Connections  of  induction-motor  coils. 

that  can  be  used  for  quarter-phase  motors  is  of  the  type  of  the 
one  just  described. 

With  three-phase  motors  the  poles  can  be  grouped  not  only 
as  previously  suggested,  but  a  variation  of  connections  from 
delta  to  star,  or  the  reverse,  can  be  made.  A  delta-connected, 


Delta  Connected  Coils.  Star  Connected  Cotb. 

FIG.  268. — Three-phase  motor  coil  connections. 

two-pole  motor  is  shown  at  the  left  in  Fig.  268,  where  the  three 
phases  are  indicated  by  the  letters,  A,  B  and  C.  Any  one  of 
these  phases  may  have  poles  connected  in  either  series  or 
multiple.  In  a  delta  connection  with  the  coils  spaced  120 
deg.  apart,  as  shown  in  Fig.  268,  each  phase  has  the  line 
voltage  E. 


268  ELECTRICAL  MACHINERY  [ART.  406 

In  the  star  connection  the  phases  are  joined  as  shown  at  the 
right  in  Fig.  268.  In  this  case,  as  with  the  delta  connection 
each  phase  may  have  poles  in  series  or  in  multiple.  In  the 
star  connection  of  Fig.  268,  each  coil  has  a  voltage  of  0.58 
XE. 

406.  Winding  Faults  of  Induction  Motors. — When  a  new 
induction  motor  is  received,  it  sometimes  happens  that  in 
attempting   to   operate   the  machine,  although  it  will  start, 
the  currents  are  excessive  and  unbalanced,  undue  heating  ap- 
pears or  a  peculiar  noise  is  emitted  and  accompanied  possibly 
by  dimming  of  the  lights  on  the  same  circuit  and  the  lower- 
ing of  speed  with  perhaps  actual  shutdown  of  other  indue- 
tion  motors  thereon.     If,  after  examination,  there  is  found 
to  be  no  difficulty  with  the  air  gap,  belt  tension,  starting  resist- 
ance or  bearings,  the  probabilities  are  that  the  coils  of  the 
motor  have  been  wrongly  connected  or  that  the  winding  has 
been  damaged  during  transportation.     Certain  indications  of 
these   conditions  are  shown   by  instrument  readings.     The 
winding  faults  in  a  three-phase  motor  may  be: 

(1)  One  coil  of  the  rotor  may  be  open-circuited.  The 
armature  or  rotor  may  have  a  defective  winding  just  as  may 
the  field.  (A  coil-wound  rotor  construction  is  used  only  when 
a  starting  resistance  is  used.  When  a  compensator  is  used 
no  starting  resistance  is  required,  and  the  winding  consists 
simply  of  bars  connected  at  the  ends  by  a  ring.)  (2)  Two 
coils  or  phases  of  the  armature  may  be  open-circuited.  (3) 
Armature  may  be  connected  properly  but  field  coil  or  phase 
may  be  reversed.  (4)  Part  of  field  may  be  short-circuited. 
(5)  One  phase  of  field  may  be  open-circuited. 

407.  With  an  Open  Circuit  in  Field  or  Stator  in  a  three- 
phase  motor,  current  would  flow  only  in  two  legs.     There 
would  be  no  current  in  the  other  leg  and  the  motor  would  not 
start  from  rest  with  all  switches  closed.     However,  a  three- 
phase   motor   or   a   two-phase  motor  will  run  and  do  work 
single-phase  if  it  is  assisted  in  starting.     The  starting  torque 
is  zero,  but  as  the  speed  increased  the  torque  increases.     With 
a  small  motor,  giving  a  pull  on  the  belt  will  introduce  enough 
torque  so  that  it  will  pick  up  its  load.     Therefore,  while  an 


SEC.  11]     TROUBLES  OF  GENERATORS  AND  MOTORS  269 

open  circuit  in  the  field  winding  should  be  found  and  repaired, 
if  there  is  not  time  for  repairs,  the  motor  can  be  operated 
single-phase  to  about  two-thirds  of  normal  load.  The  power- 
factor  conditions  and  effects  on  the  rest  of  the  circuit  are  prac- 
tically no  worse  than  when  the  motor  is  running  three-phase. 
The  torque  graph  of  a  1-h.p.,  three-phase  induction  motor 
from  rest  to  synchronism,  when  running  single-phase,  is  indi- 
cated in  Fig.  203.  The  torque  curve  of  a  20-h.p.,  three-phase 
motor  is  given  in  Fig.  204,  and  of  a  1-h.p.,  three-phase  motor 
in  Fig.  205. 

408.  Balking  of  Induction  Motors. — With  induction  motors 
having  certain  slot  relations  between  armature  and  field,  at  one 
certain  percentage  of  speed,  the  torque  will  decrease  to  almost 
zero.     The  motor  will  start  its  load  properly,  but  will  sud- 
denly lose  its  torque  at  some  slow  speed,  perhaps  one-tenth 
normal.     Such  trouble  may  be  caused  by  a  magnetic  locking 
effect  of  the  teeth  of  the  armature  with  the  poles  of  the  field. 
This  phenomenon  cannot  easily  be  measured  with  ordinary 
measuring  instruments  and  facilities.     But  with  special  torque 
measuring  instruments  the  peculiar  synchronous  locking  can 
be  measured  and  exactly  located.     If  all  other  investigations 
show  no  cause  of  weak  torque  during  the  rise  of  the  speed 
from  rest  to  synchronism,  the  relation  between  the  number 
of  poles  and  slots  in  the  rotor  may  account  for  the  trouble. 
This  is  an  unusual  condition,  but  on  squirrel-cage  motors 
it  has  existed.     There  is  no  remedy  but  a  change  in  design, 
so  that  the  manufacturer  must  take  action  for  correction. 

409.  Squirrel-cage  Armature  or  Rotor  Troubles. — Unusual 
operation  due  to  reversals  of  phase,  phases  open-circuited, 
and  other  causes,  occur  with  squirrel-cage  armatures  as  well 
as  with  wound  armatures.      Poor  soldering  of  the  armature 
bars  may  be  the  cause.     Sometimes  a  solder  flux  may  be  used 
that  will  insure  proper  operation  for  a  while,  but  time  will 
develop  poor  electrical  contacts  due  to  chemical  action  at 
the  joints.     If  the  resistances  of  all  of  the  squirrel-cage  joints 
are  uniformly  high,  the  effect  is  simply  like  that  of  an  arma- 
ture having  a  high  resistance,  which  causes  a  lowering  of  the 
speed  and  local  heating  at  the  joints.     If  some  of  the  joints 


270  ELECTRICAL  MACHINERY  [ART.  410 

are  perfect,  but  some  bad,  the  motor  may  not  have  the 
ability  to  come  up  to  speed  and  there  will  be  unbalanced 
currents. 

410.  Effects  of  Unbalanced  Voltages  on  Induction  Motors. 
— The  maximum  output  of  a  polyphase  induction  motor  may 
be  materially  decreased  if  the  voltages  impressed  on  the  dif- 
ferent phases  are  unequal.     On  a  three-phase  system,  the 
three  voltages  between  the  legs  1-2,  2-3  and  1-3  should  be 
approximately  equal.     Also  on  a  two-phase  the  voltage  1-2 
should  equal  3-4.     If  these  voltages,  impressed  on  the  induc- 
tion motor,  are  not  equal  the  maximum  output  of  the  motor 
as  well  as  the  current  in  the  various  legs  is  proportionately 
affected. 

EXAMPLES. — With  a  two-phase  motor,  if  the  voltages  in  the  two  legs 
differ  by  20  per  cent.,  a  condition  sometimes  encountered  in  normal 
practice,  the  "output  of  the  motor  may  be  reduced  25  per  cent.  Then, 
instead  of  being  able  to  give  its  maximum  output  of,  say,  150  per  cent, 
for  a  few  moments,  it  will  give  but  112  per  cent.  The  varying  loads 
which  the  motor  may  have  to  carry  may  shut  it  down.  In  cases  of 
low  maximum  output,  the  relative  voltages  on  the  various  legs  should 
always  be  investigated.  If  they  vary,  the  trouble  may  be  due  to  this 
variation. 

In  addition  to  the  effect  on  the  maximum  output,  the  unequal  distri- 
bution of  current  in  a  two-phase  motor  under  such  conditions  may  be 
quite  serious.  Consider  a  specific  case  of  a  15-h.p.,  6-pole,  1,200- 
r.p.m.,  220-volt  motor,  with  the  voltage  on  one  leg  220  and  the  voltage 
on  the  other  leg  180;  current  in  leg  No.  1  was  60  amp.  and  in  leg  No. 
2,  35  amp.  at  full-load.  The  normal  current  at  full-load  was  35  amp. 
Thus  the  fuse  might  blow  in  the  phase  carrying  the  high  current,  caus- 
ing the  motor  to  run  single-phase.  If  an  attempt  is  made  to  start  the 
motor,  the  blown  fuse  not  being  noticed,  there  would  be  no  starting 
torque. 

Consider  the  specific  case  of  a  six-pole,  10-h.p.,  1,200-r.p.m.,  160-volt, 
three-phase  motor.  The  motor  on  normal  voltage,  at  full-load,  took  110 
amp.  in  each  leg.  With  unbalanced  voltages  of  161,  196  and  168,  only 
full-load  could  be  carried,  although  the  average  of  these  voltages  is  such 
that  it  might  be  assumed  that  25  per  cent,  overload  should  be  carried. 

411.  Induction  Motor  Starting  Compensator  Troubles. — 

Sometimes  a  mistake  is  made  in  the  connections  to  the  com- 
pensator, so  that  full  voltage  is  used  at  starting  and  the 
lesser  voltage  after  throwing  over  the  switch.  Then  the  motor 


SEC.  11]    TROUBLES  OF  GENERATORS  AND  MOTORS  271 

at  starting  takes  excessive  current,  and,  since  the  maximum 
output  is  in  proportion  to  the  square  of  the  voltage,  the  motor 
capacity  is  much  reduced  when  it  is  apparently  running  on 
the  operating  position.  Such  action,  therefore,  can  usually  be 
accounted  for  by  a  wrong  connection  in  the  compensator. 
Sometimes  a  motor  connected  to  a  compensator  takes  more 
current  at  starting  than  it  should,  under  which  conditions  a 
lower  tap  should  be  tried.  Compensators  are  usually  sup- 
plied with  various  taps  and  the  one  should  be  selected  which 
produces  the  least  disturbance  on  the  line,  giving  at  the  same 
time  the  desired  starting  torque  on  the  motor. 

412.  When  a  Motor,  Having  Been  Connected  to  a  Com- 
pensator, Will  Not  Start,  the  cause  may  be  entirely  in  the 
compensator.     The  compensator  may  have  become  open-cir- 
cuited, due  to  a  flash  within.     The  switch  may  have  become 
deranged,  so  that  it  will  not  close,  or  a  connection  within 
the  compensator  may  have  become  loosened.     Possibly,  when 
a  motor  will  not  start  when  connected  to  a  compensator  just 
installed,  a  secondary  coil  may  be  " bucked"  against  another 
secondary  coil  within  the  compensator  so  that  on  voltage  is 
produced  by  the  compensator  at  the  motor.     This  results  in 
no  appreciable  excess  heating  and  in  no  apparent  phenomenon 
which  would  account  for  the  motor  not  starting.     An  am- 
meter in  the  motor  leads  will  indicate  the  absence  of  current, 
or  a  voltmeter  will  indicate  the  absence  of  voltage. 

413.  Induction-motor  Collector-ring  Troubles. — It  is  essen- 
tial that  the  contact  of  the  brushes  on  the  collector  rings 
be  good,  else  the  contact  resistance  will  be  so  great  as  to  slow 
the  motor  down  and  to  cause  heating  of  the  collector  itself. 
This  effect  is  particularly  noticeable  when  carbon  brushes  are 
used.     The  contact  resistance  of  a  carbon  brush  under  normal 
operation  pressure  and  carrying  its  usual  density  of  current 
(40  amp.  per  sq.  in.)  is  0.04  ohm  per  sq.  in.     Thus,  under 
normal  conditions,  the  drop  is  0.04  X  40,  which  equals  1.6 
volts.     If  the   contact  is  only  one-quarter  the  surface,  this 
drop  would  be  6.4  volts,  and  might  materially  affect  the  speed 
of  the  motor.     Thus,  if  the  speed  is  below  synchronous  speed 
more  than  it  should  be  (normally  it  should  not  be  over  4  per 


272  ELECTRICAL  MACHINERY  [ART.  414 

cent,  below),  an  investigation  of  the  fit  of  the  brush  upon 
the  collector  may  show  up  the  trouble. 

If  copper  brushes  are  used,  this  trouble  is  much  less  liable 
to  occur,  since  the  drop  of  voltage,  due  to  contact  resistance 
when  running  at  normal  density  (150  amp.  per  sq.  in.),  is  only 
one-tenth  that  of  carbon.  The  same  trouble  may  occur  due 
to  the  pigtail,  which  is  usually  used  with  carbon  brushes, 
making  poor  contact  with  the  carbon,  which  gives  the  same 
effect  as  a  poor  contact  with  the  collector  itself. 

414.  Hunting  of  Induction  Motors. — In  very  rare  cases  an 
induction  motor  will  hunt  and  cause  much   trouble.     The 
phenomenon  appears  as  a  speed  variation  of  1  or  2  per  cent, 
each  side  of  the  normal  speed,  with  a  period  of  vibration  de- 
pending upon  the  conditions.     It  may  be  anywhere  from  10 
to  500  swings  a  minute.     This  rare  phenomenon  of  induction 
motors  depends  upon  the  drop  in  the  line  between  the  genera- 
tor operating  the  induction  motor  and  the  motor  itself,  and 
upon  the  design  and  slot  relations  of  field  and  armature.     It 
will  cease  if  the  line  resistance  be  cut  out  between  the 'motor 
and  the  generator.     If  this  is  not  possible,  it  can  sometimes 
be  stopped  on  at  three-phase  motor  by  changing  from  delta 
to  Y  connection,  or  possibly  the  grouping  of  the  poles  may  be 
changed.     In  any  case,  the  flux  in  the  motor  is  altered. 

415.  The  Period  of  Motor  Hunting  Has  Nothing  Whatever 
To  Do  with  the  Hunting  of  the  Generator. — Hunting  of  a  motor 
may  occur  even  though  the  generator  speed  is  exactly  uniform. 
This  action  is  entirely  distinct  from  a  variation  of  the  uni- 
formity of  the  speed  of  the  generator  due  to  the  engine  driv- 
ing (Art.  289)  which  lack  of  uniformity  is  repeated  by  the  motor 
itself.     It  is  more  vicious  and  usually  results  in  a  gradual  in- 
crease of  amplitude  of  swing  until  the  motor  finally  gets  swing- 
ing so  badly  that  it  finally  breaks  down  and  stops  entirely. 
Ordinarily,  the  manufacturer  is  responsible,  but  a  change  of 
connections  will  often  cure  the  trouble  and  keep  the  apparatus 
in  operation  until  a  permanent  correction  can  be  effected. 

416.  Improper  End  Play  in  Induction  Motors. — Induction 
motors  are  so  designed  that  the  revolving  parts  will  play  end- 
wise in  the  bearing;  Jf6  in.  or  so.     If  in  setting  up  the  ma- 


SEC.  11]    TROUBLES  OF  GENERATORS  AND  MOTORS  273 

chine  the  bearings  so  limit  this  end  action  that  the  rotor  does 
not  lie  exactly  in  the  middle  of  the  stator,  there  is  a  strong 
magnetic  pull  tending  to  center  the  rotor.  If  the  bearings 
will  not  permit  this  centering,  the  thrust  collars  must  take 
the  extra  thrust  which,  in  an  induction  motor,  is  considerable. 
If  in  addition  to  the  magnetic  thrust  the  belt  pull  is  such  as 
to  also  draw  in  the  same  direction,  the  trouble  is  aggravated. 
The  end  force  may  be  such  as  to  heat  the  bearing  excessively 
and  to  cause  cutting,  soon  rendering  the  motor  inoperative. 

417.  In  Case  of  Trouble  with  Bearings,  the  end  play  should 
be  tested  by  pushing  against  the  shaft  with  a  small  piece  of 
wood,  placed  on  the  shaft  center.     With  the  machine  operat- 
ing and  rotating  under  normal  conditions  there  should  be  no 
particular  difficulty  in  pushing  the  shaft  first  one  way  from 
one  side,  and  then  the  other  way  from  the  other  side.     If  it 
is  found  that  the  revolving  part  is  hugging  closely  against  one 
side,  the  trouble  can  be  corrected  either  by  pressing  the  spider 
along  the  shaft  in  a  direction  toward  which  the  hugging  is 
occurring,  or  by  driving  the  tops  of  the  lamination  teeth  in  the 
same  direction.     With  a  wooden  wedge,  the  tops  of  the  teeth 
can  often  without  any  difficulty  be  driven  over  J^  to  %$  in. 
This  movement  will  usually  correct  the  trouble.     Driving  the 
teeth  of  the  stator  %  in.  or  so  in  the  opposite  direction  to 
that  of  the  end  thrust  will  usually  accomplish  the  same  result. 
It  is  best  to  choose  the  teeth  (stator  or  rotor)  which  are  most 
easily  driven  over.     The  thin  long  ones  move  easier  than  do 
the  short  broad  ones. 

418.  Oil  Leakage  of  Induction  Motor  Bearings. — Some- 
times a  bearing  will  permit  oil  to  be  drawn  out,  perhaps  a  very 
little  at  a  time.     Ultimately  enough  will  accumulate  to  show 
on  the  outside  or  on  the  windings  of  the  machine.     While  a 
motor  will  run  for  a  period  with  its  windings  wet  with  ordinary 
lubricating  oil  without  being  apparently  injured,  insulation 
soaked  with  oil  will  deteriorate  and  eventually  fail.     One  of 
the  principal  causes  is  a  suction  of  the  oil  due  to  the  drafts 
of  air  from  the  rotor,  and  one  of  the  best  methods  of  stopping 
the  trouble,  under  ordinary  conditions,  is  to  cut  grooves  in 
the  babbitt  lining  as  shown  in  Fig.  269  at  B  and  D.     These 

18 


274 


ELECTRICAL  MACHINERY 


[ART.  419 


grooves  on  a  50-h.p.  motor  may  be  J£  in.  deep  and  Jf  g  in. 
wide.  Each  groove  has  three  holes  drilled  through  the  bearing 
shell  to  convey  the  oil  collected  by  the  grooves  into  the  oil  well. 
These  grooves  are  just  as  effective  with  a  split  as  with  a  solid 
bearing.  It  is  impossible  here  to  go  into  the  various  causes  of 
oil  leakage.  The  grooves  as  suggested  are  a  general  remedy  and 
may  correct  many  oil-leakage  difficulties.  See  Art.  233  for  in- 
formation relating  to  ball  bearings  for  electrical  machinery. 

419.  To  Locate  a  Short-circuited  Coil  in  an  Alternating- 
current  Motor  or  Generator  there  are  several  methods  which 
may  be  used.  Which  one  is  most  applicable  is  determined  by 
the  conditions  of  the  case.  Frequently  a  short-circuited  coil 


'Sheet  Irvn  Feeler 


•Exploring' 
Coil 


FIG.  269. — Grooves  to  prevent  oil        FIG.  270. — An  "inducer"  or  ex- 
leakage,  ploring-coil    for     locating     short- 
circuited  coils. 

will  "burn  out,"  that  is,  the  insulation  will  be  charred  com- 
pletely from  it,  in  which  case  the  identification  is  obvious. 
When  this  occur?  it  is  usually  possible  to  cut  out  the  short- 
circuited  coil,  to  close  the  circuit  and  continue  operation. 
Often  such  a  coil  will  not  become  sufficiently  heated  to  burn 
out,  but  can  be  located  by  feeling  with  the  hand  because  of  its 
excess  temperature  above  that  of  the  adjacent  coils.  Where 
.a  motor  is  under  consideration,  the  machine  can  be  operated 
for  a  time  until  the  short-circuited  coil  heats  so  that  it  can  be 
located  with  the  hand  and  then  it  should  be  marked  with  a 
piece  of  chalk  for  future  identification. 

420.  An  "Inducer"  for  Locating  Short-circuited  Coils*  is 
illustrated  in  Fig.  270.     This  device,  the  principle  of  operation 

*  ELECTRICAL  REVIEW,  Aug.  29,  1914,  p.  425. 


SEC.  11]    TROUBLES  OF  GENERATORS  AND  MOTORS  275 

of  which  is  similar  to  that  of  the  one  described  in  Art.  270, 
has  been  used  successfully  for  locating  short-circuited  coils. 
A  C-shaped  soft-iron  core,  N  (preferably  laminated)  is  pro- 
vided with  a  winding  having  sufficient  reactance  so  that  when 
the  coil  is  connected  across  the  available  (a  110- volt  line  will 
do)  alternating-current  line,  it  will  not  overheat.  The  coil 
may  otherwise  be  of  any  reasonable  proportions.  When  this 
exploring  coil  is  placed  over  the  core  laminations,  A,  and  rests 
over  a  slot  in  which  one  side  of  a  short-circuited  coil  lies,  the 
exploring  coil  will  induce  sufficient  current  in  this  short-cir- 
cuited coil  to  set  up  a  magnetic  flux  across  the  slot,  S  (Fig. 
270),  which  can  be  detected  with  a  soft  sheet-iron  feeler,  F. 
Hence,  to  use  an  exploring  coil  of  this  type,  shift  the  energized 
coil  around  the  entire  interior  circumference  of  the  state,  at  the 
same  time  holding  the  feeler  at  a  distance  from  the  gap  of  the 
exploring  coil,  equal  to  the  pitch  of  the  coils. 


SECTION    12 

TESTING  OF  ALTERNATING-CURRENT  GENERATORS 
AND   MOTORS 

421.  An  Alternating -current  Generator  Excitation  or  Mag- 
netization Test  is  diagrammed  in  Fig.  271.  The  object  of 
this  test  is  to  determine  the  change  of  the  armature  voltage, 
due  to  the  variation  of  the  field  current,  when  the  external 
circuit  is  open.  As  shown  in  Fig.  271  the  field  circuit  is  con- 
nected with  an  ammeter,  I,  and  a  rheostat,  R,  in  series  with 
a  direct-current  source  of  supply.  The  resistance  of  the  rheo- 
stat is  varied,  and  readings  of  the  voltmeters  across  the  arma- 
ture coils  and  of  the  ammeter,  are  recorded.  The  generator 
speed  must  be  kept  constant,  preferably  at  the  normal  speed 


Three-Phase  Generator-^- 


FIG.  271. — Arrangement  of  apparatus  for  determining  data  for  plot- 
ting a  magnetization  graph  of  a  three-phase,  alternating-current 
generator. 

which  is  specified  on  the  nameplate.  By  plotting  the  current 
and  the  voltage  values  thus  obtained  on  squared  paper  the 
excitation  or  magnetization  graph  of  the  machine  will  be  the 
result. 

422.  A  Synchronous-impedance  Test  of  an  Alternating- 
current  Generator  may  be  made  as  indicated  in  Fig.  272. 
In  determining  the  regulation  of  an  alternating-current  gen- 
erator, it  is  necessary  to  obtain  what  is  called  the  "synchro- 
nous impedance"  of  the  machine.  To  do  this,  the  field  is 
connected,  as  shown.  The  voltmeters  (Ei,  Ez  and  Es)  are 

276 


SEC.  12]      TESTING  OF  GENERATORS  AND  MOTORS 


277 


removed  and  the  armature  short-circuited  with  the  ammeters 
(Ji,  J2,  and  73)  in  circuit.  The  field  current,  //,  is  then  varied, 
the  armature  driven  at  synchronous  speed,  and  the  arma- 
ture current  measured  by  the  ammeters.  The  relation  be- 
tween field  and  armature  amperes  is  then  plotted  on  squared 
paper.  A  combination  of  the  results  of  this  test,  with  those 
obtained  from  the  test  shown  in  Fig.  271,  is  used  in  the  deter- 
mination of  the  regulation  of  a  generator.  Engineers  differ, 
however,  as  to  application  of  the  values  obtained  as  above 
to  the  determination  of  regulation.  Methods  of  combining 
the  results  will  not,  therefore,  be  discussed  here. 

423.  A  Load  Test  of  a  Three-phase,  Alternating-current 
Generator  may  be  made  by  means  of  the  connection  shown 


FIG.  272. — Equipment  for  making  a  load  test  on  a  three-phase  alternat- 
ing current  generator. 

in  Fig.  272.  Readings  of  armature  current  and  field  amperes 
are  obtained  at  any  desired  load.  The  field  current,  //,  can 
be  varied  also  so  as  to  maintain  constant  armature  voltage 
irrespective  of  load,  or  the  field  current  may  be  kept  constant 
and  the  armature  voltage  allowed  to  vary  as  the  load  in- 
creases. The  connections  may  also  be  used  to  make  a  tem- 
perature test  on  the  generator  by  loading  it  with  an  artificial 
load.  In  some  cases  after  the  generator  is  installed  the  ar- 
rangement shown  may  be  used  to  make  a  temperature  test, 
loading  the  machine  with  the  actual  commercial  load  the 
generator  is  serving. 

424.  In  Testing  Alternating-current  Generators  for  Insu- 
lation Resistance,  the  same  general  methods  (Art.  245)  may 
be  followed  as  with  direct-current  machines. 


278 


ELECTRICAL  MACHINERY 


[ART.  425 


425.  For  Determining  the  Brake  Horse -power  Output  and 
the  Torque  of  Alternating-current  Motors  the  same  Prony- 
brake  methods  (See  Art.  237)  that  are  used  with  direct-current 
motors  are  employed.     Refer  to  Sec.  5,  "  Testing  of  Direct- 
current  Motors  and  Generators,"  for  further  information. 

426.  With  Very  Large  Machines  Under  Test,  it  is  inadvis- 
able to  use  the  above  method  as  it  is  sometimes  difficult  to 
so  adjust  the  pulleys  and  belt  tension  that  the  belt  slip  will 
be  just  right  to  compensate  for  the  difference  in  the  diameters 
of  the  pulleys,  and  very  violent  flapping  of  the  belt  results. 
To  meet  such  conditions  various  other  methods  have  been 
devised.     One  which  gives  consistent  results  is  the  following: 


Armature  ez       •     o* 
Coi/<;    <&       >A- Stator 
'"*-*>        I  (Stationary 
,    Armature) 


V^T  , . x ''(Revolving Field)  \        ^  ^       / 
^Three -Phase  Generator-^- -^ .._-'" 

FIG.  273. — Equipment  for  making  a  temperature  test  of  a  large  three- 
phase  generator  or  synchronous  motor. 


Supply  the  rotor  with  normal  field  current.  The  stator,  S,  is 
connected  in  open  delta  (Fig.  273)  and  full-load  current  sent 
through  it  from  an  external  source  of  direct  current.  Care 
should  be  taken  to  ground  one  terminal  of  the  direct-current 
generator,  as  at  G,  so  as  to  eliminate  danger  of  shock,  to  at- 
tendants, due  to  the  voltage  on  the  stator  winding.  The 
rotor  is  then  driven  at  synchronous  speed. 

If  the  stator  is  designed  for  2,300-volt  star  connection,  the 
voltage  generated  in  each  leg  of  the  delta  will  be  1,330  volts, 
and  unless  one  leg  of  the  direct-current  generator  were 
grounded,  the  tester  might  receive  a  severe  shock  by  con- 
tacting with  the  direct-current  circuit.  The  insulation  of  the 


SEC.  12]      TESTING  OF  GENERATORS  AND  MOTORS 


279 


direct-current  machine  will  also  be  subjected  to  abnormal 
strain  unless  one  terminal  is  grounded. 

By  the  above  method  the  rotor  is  subjected  to  its  full 
copper  loss  and  the  stator  to  full  copper  loss  and  core  loss. 
Temperature  readings  are  taken  as  recommended  in  the 
STANDARDIZATION  RULES  OF  THE  A.  I.  E.  E.  This  method 
may  also  be  used  with  satisfactory  results  on  large  three- 
phase  motors  of  the  wound-rotor  type. 

427.  The  Method  of  Measuring  the  Input  of  a  Single- 
phase  Motor  of  any  type  is  shown  in  Fig.  274,  the  ammeter, 
voltmeter  and  wattmeter  being  connected  as  indicated.  The 
ammeter,  A,  measures  the  current  flowing  through  the  motor, 
the  voltmeter,  C,  the  e.m.f.  across  the  terminals  of  the  motor, 
and  the  wattmeter,  By  the  total  power  which  flows  through 


FIG.  274. — Instruments  and  connections  for  measuring  the  input  of  a 
single  phase  motor. 

the  motor  circuit.  With  the  connections  as  shown,  the  watt- 
meter would  also  measure  the  slight  losses  in  the  voltmeter 
and  the  potential  coil  of  the  wattmeter,  but  for  motors  of 
Y±  h.p.  and  larger,  this  loss  is  so  small  that  it  may  be  neg- 
lected. The  power-factor  may  be  calculated  by  dividing  the 
true  watts  as  indicated  by  the  wattmeter,  by  the  product  of 
the  volts  and  amperes. 

428.  A  Three-phase  Generator  or  Synchronous-motor 
Temperature  Test  may  be  made  as  diagrammed  in  Fig.  275, 
which  shows  the  arrangement  usually  employed  in  shop  tem- 
perature tests  of  these  machines.  The  two  generators  or  syn- 
chronous motors  of  same  size  and  type  are  belted  together, 
one,  M ,  to  be  driven  as  a  synchronous  motor  and  the  other, 
G,  as  an  alternating-current  generator.  The  method  employed 
is  to  synchronize  the  synchronous  motor,  M,  with  the  generator 


280 


ELECTRICAL  MACHINERY 


[ART.  429 


or  generators  on  the  three-phase  circuit,  L,  and  then  connect 
it  to  the  line  by  means  of  a  three-pole,  single-throw  switch. 
The  alternating-current  generator,  G,  is  then  similarly  syn- 
chronized with  the  generator  of  the  three-phase  circuit  and 
thrown  on  the  line.  By  varying  the  field  of  the  generator  it 
can  be  made  to  carry  approximately  full-load.  The  motor 
will  then  also  be  approximately  fully  loaded.  The  usual 
method  is  to  have  the  motor  carry  slightly  in  excess  of  full- 
load,  and  the  generator  slightly  less  than  full-load.  Under 
these  conditions  the  motor  will  run  a  little  warme4r  than  it 
should  with  normal  load,  while  the  generator  will  run  slightly 


-Ammeter 


)  5)^'  'Circuit  Breakers  Circuit  Breakers-''"'^ 


i^'Three- Phase  Line 


..-Wattmeter 


,»--Pz  J    ^Three-Phase  Synchronous  Motor 


Stationary 
Armatur' 


/      /Revolving  Field 

D/~  Field  Ammeter       ..-Belt 


Direct-Current  Supply  Line^ 


FIG.  275. — Method  of  making  a  temperature  test  of  a  three-phase  gen- 
erator or  motor. 

cooler.  Temperature  measurements  are  then  made  in  the  same 
way  as  discussed  under  three-phase  motors.  The  necessary 
ammeters,  voltmeters  and  wattmeters  for  adjusting  the  loads 
on  the  motors  and  generator  are  shown  in  the  illustration. 

If  the  pulleys  are  of  sufficient  size  to  transmit  the  full-load 
with,  say,  1  per  cent,  slip,  the  pulley  on  the  motor  should  be 
1  per  cent,  larger  in  diameter  than  the  pulley  on  the  generator, 
so  that  the  generator  will  remain  in  synchronism  and,  at  the 
same  time,  deliver  power  to  the  circuit,  L. 

429.  To  Determine  the  Approximate  Input  Load  on  a  Three- 
phase  Motor  by  the  Voltmeter  and  Ammeter  Method  the  ar- 


SEC.  12]      TESTING  OF  GENERATORS  AND  MOTORS 


281 


rangement  shown  in  Fig.  276  may  be  used.  The  current 
through  one  of  the  three  lines  and  the  voltage  across  one  phase 
is  measured.  If  the  voltage  is  approximately  the  rated  voltage 
of  the  motor  and  the  amperes  the  rated  current  of  the  motor 
(as  noted  on  the  nameplate),  it  may  be  assumed  that  the 
motor  is  carrying  approximately  full-load.  If,  on  the  other 
hand,  the  amperes  are  much  in  excess  of  full-load  rating,  it  is 
evident  that  the  motor  is  carrying  an  overload.  The  heat 
generated  in  the  copper  varies  as  the  square  of  the  current. 
That  generated  in  the  iron  varies  anywhere  from  the  1.6  power 
to  the  square  of  the  current.  This  method  is  exceedingly 
convenient  if  a  wattmeter  is  not  available,  although  it  is,  of 
course,  of  no  value  for  the  determination  of  the  efficiency  or 
power-factor  of  the  apparatus.  This  method  gives  fairly 


Three-Phase  Motor  Under  Test* 


FIG.  276. — Connections  for  determining  the  load  on  a  three-phase  motor 
by  the  approximate  "voltmeter-and-ammeter"  method. 

accurate  results,  providing  the  load  is  fairly  well  balanced  on 
all  three  of  the  phases  of  the  motor.  If  there  is  much  difference 
in  the  voltages  across  the  three  phases,  the  ammeter  should  be 
switched  from  one  circuit  to  another,  and  the  current  measured 
in  each  phase.  If  the  motor  is  very  lightly  loaded  and  the 
voltage  of  the  different  phases  varies  by  2  or  3  per  cent.,  the 
current  in  the  three  legs  of  the  circuit  will  vary  20  to  30  per 
cent. 

430.  Where  an  Accurate  Input  Test  of  a  Three-phase 
Motor  is  Desired  the  "Two -wattmeter"  Method  is  Used. — 
This  is  illustrated  in  Fig.  277.  Assume  that  the  motor  is 
loaded  with  a  Prony  brake  so  that  its  output  can  be  deter- 
mined. This  method  gives  correct  results  even  with  consider- 
able unbalancing  in  the  voltages  of  the  three  phases.  With 


282 


ELECTRICAL  MACHINERY 


[ART.  431 


the  connections  as  shown,  the  sum  of  the  two  wattmeter  read- 
ings gives  the  total  power  in  the  circuit.  Neither  meter  by 
itself  measures  the  power  in  any  one  of  the  three  phases.  In 
fact,  with  light-load  one  of  the  meters  will  probably  give  a 
negative  reading,  and  it  will  then  be  necessary  to  either  re- 
verse its  current  or  potential  leads  in  order  that  the  deflec- 
tion may  be  noted.  In  such  cases  the  algebraic  sums  of  the 
two  readings  must  be  taken.  In  other  words,  if  one  reads  + 
500  watts  and  the  other  —  300  watts,  the  total  power  in 
the  circuit  will  be:  500  -  300  =  200  watts. 

As  the  load  comes  on,  the  readings  of  the  instrument  which 
gave  the  negative  deflection  will  decrease  until  they  drop  to 
zero,  and  it  will  then  be  necessary  to  again  reverse  the  po- 


'Ammeters  ^Wattmeters         '^Voltmeters 

FIG.  277. — Connections  of  instruments  for  determining  the  load  on  a 
three-phase  induction  motor  by  the  "two-wattmeter"  method. 

tential  leads  on  this  wattmeter.  Thereafter,  the  readings  of 
both  instruments  will  be  positive,  and  the  numerical  sum  of 
the  two  should  be  taken  as  the  measurement  of  the  load.  If 
one  set  of  the  instruments  is  removed  from  the  circuit,  the 
reading  of  the  remaining  wattmeter  will  have  absolutely  no 
meaning.  As  suggested  above,  it  will  not  indicate  the  power 
under  these  conditions  in  any  one  phase  of  the  circuit.  The 
power-factor  is  obtained  by  dividing  the  actual  watts  input  by 
the  product  of  the  average  of  the  voltmeter  readings  X  the  aver- 
age of  the  ampere  readings  X  1.73. 

431.  The  Three-phase-motor  Input  Test,  Polyphase-watt- 
meter Method  is  identical  with  that  described  above  except 
that  the  polyphase  wattmeter  itself  combines  the  movements 
of  the  two  wattmeters.  Otherwise  the  method  of  making  the 


SEC.  12]      TESTING  OF  GENERATORS  AND  MOTORS  283 

measurements  is  identical.  If  the  power-factor  is  known  to  be 
less  than  50  per  cent.,  connect  one  of  the  wattmeter  movements 
so  as  to  give  a  positive  deflection;  then  disconnect  move- 
ment 1  and  connect  movement  2  so  as  to  give  a  positive  de- 
flection. Then  reverse  either  the  potential  or  current  leads 
of  the  movement  giving  the  smaller  deflection,  leaving  the 
remaining  movement  with  the  original  connection.  The  read- 
ings now  obtained  will  be  the  correct  total  watts  delivered 
to  the  motor. 

If  the  power-factor  is  known  to  be  over  50  per  cent.,  the 
same  method  should  be  employed,  except  that  both  movements 
should  be  independently  connected  to  give  positive  readings. 
An  unloaded  induction  motor  has  a  power-factor  of  less  than 
50  per  cent.,  and  may,  therefore  be  used  as  above  for  determin- 
ing the  correct  connections.  For  a  better  understanding  of 
the  reasons  for  the  above  method  of  procedure,  it  is  suggested 
that  the  discussion  of  power  measurement  by  the  two-watt- 
meter method  (Art.  430) -be  read. 

The  power-factor  may  be  calculated  as  under  the  test  pre- 
viously described  in  Art.  427.  Connect  as  per  Fig.  277.  The 
following  check  on  the  connections  may  be  made.  Let  the 
polyphase  induction  motor  run  idle,  that  is,  with  no  load. 
The  motor  will  then  operate  with  a  power-factor  less  than  50 
per  cent.  The  polyphase  meter  should  give  a  positive  indi- 
cation, but  if  each  movement  is  tried  independently  one  will 
be  found  to  give  a  negative  reading,  the  other  movement  will 
give  a  positive  reading.  The  movements  can  be  tried  inde- 
pendently by  disconnecting  one  of  the  potential  leads  from 
the  binding  post  of  one  movement.  When  the  power-factor 
is  above  50  per  cent,  then  both  movements  will  give  positive 
deflection. 

432.  The  "One-wattmeter-method,"  Three -phase -motor 
Input  Test  is  equivalent  to  the  two-wattmeter  method  with 
the  following  difference.  A  single  voltmeter  (Fig.  278)  with 
a  switch,  A,  can  be  used  to  connect  the  voltmeter  across 
either  one  of  two  phases.  Three  switches,  B,  C  and  D, 
are  employed  for  changing  the  connection  of  the  ammeter 
and  wattmeter  in  either  one  of  the  two  lines.  With  the 


284 


ELECTRICAL  MACHINERY 


[ART.  443 


switches  B  and  D  in  the  position  shown,  the  ammeter  and 
wattmeter  series  coils  are  connected  in  the  upper  line.  The 
switch  C  must  be  closed  under  these  conditions  to  close  the 
middle  line.  Another  reading  should  then  be  taken  before 
any  change  of  load  has  occurred,  with  switch  A  thrown 
down,  switch  B  closed,  switch  D  thrown  down  and  switch 
C  open.  The  ammeter  and  the  current  coil  of  the  watt- 
meter will  then  be  connected  to  the  middle  line  of  the  motor. 
To  prevent  any  interruption  of  the  circuit,  the  switches 
B,  D  and  C  should  be  operated  in  the  order  given  above. 
With  very  light  load  on  the  motor,  the  wattmeter  will 
probably  give  a  negative  deflection  in  one  phase  or  the  other, 
and  it  will  be  necessary  to  reverse  its  connections  before 


Three-Phase  Motor  Under  Test* 


"Single-Pole  Switch 
-Three-Phase  Line 


FIG.  278.— Arrangement   of   circuits  for  testing   a   three-phase   motor 
by  the  "one-wattmeter"  method. 

taking  the  readings.  For  this  purpose  a  double-pole,  double- 
throw  switch  is  sometimes  inserted  in  the  circuit  of  the  po- 
tential coil  of  the  wattmeter  so  that  the  indications  can  be 
reversed  without  disturbing  any  of  the  connections.  It  is 
suggested,  that,  before  undertaking  this  test,  the  instructions 
for  test  by  the  two-wattmeter  (Art.  430)  and  by  the  poly- 
phase-wattmeter (Art.  431)  methods  be  read. 

433.  The  "One-wattmeter-and-Y-box"  Method  of  Testing 
the  Input  of  a  Three-phase  Motor,  a  diagram  for  which  is 
shown  in  Fig.  279,  is  of  service,  only,  where  the  voltages  of 
the  three  phases  are  the  same.  A  slight  variation  in  the 
voltages  of  the  different  phases  may  result  in  a  very  large 
error  in  the  readings  of  the  wattmeter,  and  inasmuch  as  the 
voltages  of  all  commercial  three-phase  circuits  are  more  or 


SEC.   12]     TESTING  OF  GENERATORS  AND  MOTORS 


285 


less  unbalanced,  this  method  is  not  to  be  recommended  for 
motor  testing.  With  balanced  voltages  in  all  three  phases, 
the  power  is:  that  indicated  by  the  wattmeter,  multiplied  by 
3.  Power-factor  may  be  calculated  as  described  above. 


Three -Phase  Motor  Under  Test* 


.Voltmeter 
'Ammeter       'Wattmeter  \ 


FIG.  279. — Method  of  testing  a  three-phase  motor  with  one  wattmeter 
and  a  Y  box.     (To  be  used  on  balanced  circuits  only.) 

434.  The  Method  of  Testing  for  the  Input  of  a  Three- 
phase  Motor  Where  the  Neutral  of  the  Motor  has  Been 
Brought  Out  is  diagrammed  in  Fig.  280.  Some  star-con- 
nected motors  have  a  connection  from  the  neutral  point 
brought  outside  of  the  motor  frame  of  the  stator  or  armature 


FIG.  280. — "Single-wattmeter"  method  of  testing  a  three-phase  motor 
which  has  the  neutral  brought  outside  the  frame. 

winding.  In  this  case  the  testing  circuit  may  be  connected 
as  shown.  The  voltmeter,  E,  measures  voltage  between  the 
neutral  and  one  of  the  lines,  and  the  wattmeter,  Py  the  power 
in  one  of  the  three  phases  of  the  motor.  Therefore,  the  total 
power  taken  by  the  motor  will  be  3  times  the  wattmeter  read- 
ings. By  this  method,  as  accurate  results  may  be  obtained 
^as  with  the  two-wattmeter  method.  The  power  factor  will 


286 


ELECTRICAL  MACHINERY 


[ART.  435 


be:  the  indicated  watts  divided  by  the  product  of  the  indicated 
amperes  and  volts. 

435.  The  Temperature  Test  of  a  Three-phase  Induction 
Motor  may  be  conducted  as  delineated  in  Fig.  281.  Tempera- 
ture tests  are  usually  made  on  small  induction  motors  by 
belting  the  motor  to  a  generator  and  loading  the  generator  with 
a  lamp-bank  or  resistance  until  the  motor  input  is  equal  to 
full-load  input.  If,  however,  the  motor  is  of  considerable  size 
so  that  the  cost  of  the  energy  expended  in  making  the  test 
becomes  a  large  item  in  the  expense  of  testing,  the  method 
shown  in  Fig.  206  may  be  employed.  Two  motors,  preferably 


Three -Phase  Line^ 


—Voltmeter 


{Three  ~Phct$e 
•  Motor  Running 
{•Below  Synchronous 
Speed 


Wattmeter; 


Three  -Phase 
Motor  Driven  Above 
Synchronous  Speed 
and  Operating  as 
an  Induction 
Generator* 


FIG.  281. — Apparatus  and  arrangement  thereof  for  conducting  a  tem- 
perature test  of  a  large  three-phase  induction  motor. 

of  size  the  same  and  type,  are  required.  One,  M,  is  driven  as 
a  motor  and  runs  slightly  below  synchronism,  due  to  its  slip 
when  operating  under  load.  This  motor  is  belted  to  the  second 
machine,  G.  If  the  pulley  of  the  second  machine  is  smaller 
than  the  pulley  of  the  first  the  second  will  then  operate  as  an 
induction  generator,  and  will  return  to  the  line  as  much  power 
as  the  first  motor  draws  from  the  line,  less  the  losses  of  the 
second  machine. 

By  selecting  the  ratio  of  pulleys  properly,  the  first  machine 
can  be  caused  to  draw  full-load  current  and  full-load  energy 
from  the  line.  In  this  way,  the  total  energy  consumed  is 


SEC.  12]      TESTING  OF  GENERATORS  AND  MOTORS  287 

equivalent  to  the  total  of  the  losses  of  both  machines,  which  is 
approximately  twice  the  losses  of  a  single  machine.  Fig.  277 
shows  the  connection  of  the  wattmeters*  (without  necessary 
switches)  for  reading  the  total  energy  by  the  two-wattmeter 
method.  Detailed  connections  for  the  wattmeter  are  shown 
in  Fig.  278.  It  is  usual,  in  making  temperature  tests,  to  insert 
one  or  more  thermometers  in  what  is  presumed  to  be  the 
hottest  part  of  the  winding,  one  on  the  surface  of  the  laminae 
and  one  in  the  air  duct  between  the  iron  laminae.  The  test 
should  be  continued  until  the  difference  in  temperature 
between  any  part  of  the  motor  and  the  air  reaches  a  steady 
value.  The  motor  should  then  be  stopped  and  the  tempera- 
ture of  the  rotor  also  measured.  For  the  method  of  testing 
wound-rotor  type  induction  motors  of  very  large  size,  see 
Fig.  273.  For  the  approved  procedure  in  taking  tempera- 
ture readings  and  interpreting  results,  see  the  STANDARDIZA- 
TION RULES  OF  THE  A.  I.  E.  E. 


SECTION  13 

TEST   DETERMINATION    OF   MOTOR-DRIVE    POWER 
REQUIREMENTS 

436.  It  is  Important  That  a  Reasonably  Accurate  Determina- 
tion of  the  Power  Required  to  drive  a  certain  machine  or  a 
group  of  machines  be  made  before  the  motor  for  the  drive  is 
purchased  or  ordered.     Money  expended  in  this  direction  is 
always  well  spent.     The  reason  for  this  is  that  the  tendency 
is  usually  to  select  a  motor  considerably  larger  than  is  actually 
necessary  for  a  certain  drive.     Hence,  the  difference  in  cost 
between  a  motor  that  is  selected  on  the  basis  of  an  actual  test 
and  the  one  that  probably  would  have  been  selected  had  the 
test  not  been  made,  often  represents  a  very  material  saving  in 
capital   expenditure.     Such   a   test   is   particularly   desirable 
where  a  motor  to  pull  a  group  drive  is  to  be  installed,  because 
the  power  requirement  of  a  group  of  machines  is  a  quantity 
that  is  difficult,  if  not  impossible,  of  accurate  estimation.     It 
is  the  purpose  of  this  article  to  describe  some  simple  methods 
and  apparatus  whereby  the  power  requirements  of  ordinary 
group  or  machine-tool  drives  can  be  economically  ascertained 
with  sufficient  precision  for  commercial  purposes. 

437.  Machine-tool   Builders    and    Motor    Manufacturers 
Often  Over-estimate  the  Horsepower  Required  of  a  motor 
because  their  tendency  is,  obviously,  to  "be  on  the  safe  side." 
This  is  particularly  true  of  the  tool  builders.     These  concerns 
are  often  requested  to  advise  as  to  the  proper  horse-power 
rating  of  a  motor  to  drive  a  given  machine.     Where  a  motor 
that  is  larger  than  is  necessary  is  recommended  and  installed 
the  result  is  that  it  may  operate  most  of  the  time  at  a  fraction 
of  full  load  at  correspondingly  reduced  efficiency.     In  a  large 
installation,  the  unnecessary  electrical  losses  and  the  interest 
and  depreciation  on  the  unjustified  extra  investment  may  total 
to  a  very  considerable  annual  charge. 

288 


SEC.  13]       MOTOR-DRIVE  POWER  REQUIREMENTS 


289 


438.  To  Determine  the  Actual  Power  Required  to  drive  a 
given  machine  or  load,  probably  the  best  and  simplest  method 
is  to  arrange  a  temporary  motor  belt-drive  to  the  tool  or  load 
in  question  (Fig.  282)  and  then  measure  the  input  to  this 
temporary  motor.     To  determine  the  actual  power  required 
to  drive  the  load  there  should  be  subtracted  from  the  input  of 
the  motor  (as  measured  with  a  wattmeter  or  with  a  voltmeter 
and  an  ammeter)  the  power  losses  in  the  motor,  because  motors 
are  rated  on  the  basis  of  their  brake  horse-power  outputs. 

439.  To  Determine  the  Losses 
of  the  Testing  Motor  at  various 
loads  arrange  the  motor  for  a 
Prony-brake  test  (Art.  238)  and 
thereby  ascertain  its  input  and 
output  at  various  loads.     From 
these  data,  a  graph  indicating 
the  efficiency  at  any  load  can  be 
plotted.     Then,  the  values  from 
this  graph  can  be  used  in  making 
the  correction  for  motor   effi- 
ciency suggested  in  the  preced- 
ing paragraph.     That  is,  where 
it  is  necessary  that  the  power  in- 
put of  a  machine  or  a  drive  be 
determined  quite  accurately  it 

is  essential  that  the  testing  FlG  282.— Arrangement  of  appa- 
motor  be  calibrated — its  effi-  ratus  for  testing  power  input  to 
ciency  at  different  loads  must  n 

be  known.  It  is  often  possible  to  obtain  an  efficiency  curve  of 
a  testing  motor  from  the  manufacturer  to  the  machine.  If  such 
a  graph  is  not  attainable  this  efficiency  data  may  be  ascer- 
tained by  testing  as  suggested  above.  The  power  output  of  a 
motor  at  any  instant  is  equal  to:  the  power  input  at  the  same 
instant  multiplied  by  the  efficiency  of  the  motor,  at  the  load  which 
it  is  carrying  at  that  instant. 

440.  Frequently  Such  Refinements  as  Corrections  for  Effi- 
ciency are  Considered  Unnecessary  because  the  purchaser  will 
buy  a  motor  of  a  rating  which  is  standard  with  some  manu- 

19 


290 


ELECTRICAL  MACHINERY 


[ART.  441 


facturer.  Hence,  with  small  motors — because  it  is  always  de- 
sirable to  have  a  motor  for  any  drive  at  least  large  enough — 
corrections  for  the  efficiency  of  the  test  motor  may  prove  an 
undesirable  refinement. 


•Lag  Screw  or  Bolt  to 
Hold  Motor  to  Floor 


FIG.  283. — Portable  motor  arranged  to  determine  the  power  required  by 

machine  tools. 

441.  There  are  Many  Workable  Arrangements  for  Test 
Motors. — Figs.  283  to  286  show  some  that  have  been  applied 
in  practice.  It  is  usually  desirable,  if  several  determinations 
of  power  requirements  are  to  be  made,  to  mount  the  test  motor 


FIG.  284. — Another  arrangement  for  a  truck-mounted  testing  motor. 

on  a  truck  so  that  it  can  be  transported  to  any  part  of  the  plant 
with  a  minimum  expenditure  of  time  and  labor.  After  the 
portable  test  motor  has  been  drawn  to  the  machine  or  group 
which  it  is  to  drive,  it,  or  the  platform  on  which  it  rests,  should 
be  securely  bolted  (Fig.  283)  or  braced  in  position.  In  many 


SEC.  13]       MOTOR-DRIVE  POWER  REQUIREMENTS 


291 


Step—' 

Pulley 


Bearing-' 


cases  it  is  desirable  to  arrange  a  step  pulley  or  counter  shaft  on 
the  platform  of  the  test  motor  (Figs.  285  and  286)  to  insure 
that  the  arrangement  can  be  used 
to  drive  a  machine  or  shaft  rotat- 
ing at  any  reasonable  speed. 

442.  In  Making  a  Test  of  the 
Power   Required  the   motor  is 
drawn  to  a  convenient  location 
near  the  machine,  tool  or  shaft 
and    a    temporary    belt    drive 
arranged  (Figs.  282  and  283)  be- 
tween the  driven  device  and  the 
motor.     Then   the   machine   or 
group  to  be  driven  is  subjected  to 
its  normal  cycle  of  operations  and 
in  the  meantime  the  power  input 
of  the  motor  is  measured. 

443.  In  Connecting  the  Test  Instruments  in  the  Motor  Cir- 
cuit it  is  desirable  that  provision  be  made  which  will  render 
it  unnecessary  to  unsolder  lugs  or  disconnect  leads.     This 


Timber  Frame 


FIG.  285. — A  portable  test  motor 
equipped  with  a  counter  shaft. 


Motor. 


ive  to  Machine 


.-Shaft  Hanger 


FIG.  286. — Another     form     of     portable     test-motor-and-countershaft 

equipment. 

feature  is  particularly  important  where  it  is  necessary  to  de- 
termine the  power  input  of  some  motor  which  is  already  in- 
stalled and  in  operation.  Often  the  most  expensive  and  tedi- 
ous part  of  the  work  of  making  such  a  power-input  test  of  an 


292 


ELECTRICAL  MACHINERY 


[ART.  444 


existing  motor  is  that  involved  in  connecting  the  instruments 
into  the  motor  circuit.  As  suggested  in  Fig.  287  the  most 
convenient  and  economical  method  of  connecting  the  instru- 
ment in  such  a  circuit  appears  to  be  that  involving  the  appli- 
cation of  "dummy" — fuse-connectors,  the  construction  of  cer- 
tain types  of  which  are  detailed 
in  Figs.  288  to  291.  In  Fig.  287 
a  direct-current  testing  motor 
is  shown  but  the  general  scheme 
suggested  is  quite  as  applicable 
for  three-phase-motors.  How- 
ever, for  testing  a  three-phase 
motor  two  "dummy"  fuse-con- 
nectors will  be  required  whereas 
for  direct-current  tests  only  one 
is  necessary. 

Brass  Machine 


*• Shunt  Leads 
Dummy  Fuse  Connector 


Motor  Under  Test- 


FIG.  287. —  Connections  for 
motor  testing.  (Note  that  placing 
the  switch  in  the  circuit  ahead  of 
the  permanent  cut-out  is  in  viola- 
tion of  National  Electric  Code  Rule 
23a,  Par.  77.) 


~j- -Copper  Terminal Blade  f  \ 
Brass  Connecting  /  i 
Strap  Soldered  on'' 


U- 

FIG.  288.— "Dummy-fuse"  con- 
nector for  a  101-200  amp.  National- 
electrical-code-standard.  Knife- 
blade-contact,  fuse  block. 


444.  In  Connecting  Any  Instrument  with  Dummy-fuse- 
connectors  instead  of  disconnecting  one  of  the  leads  to  the 
motor,  the  series  coil  leads  of  the  wattmeter  or  ammeter  may 
be  inserted  at  the  cut-out  by  means  of  the  dummy-fuse-con- 
nector. The  connection  is  effected  as  shown  at  Fig.  287. 
One  of  the  fuses  is  removed  from  the  cut-out  and  in  its  stead 
is  inserted  (Fig.  288)  a  dummy-fuse-connector-.  The  leads  of 
the  wattmeter  or  ammeter  are  connected — frequently  perma- 
nently—to the  binding  post  B  and  Bl  (Fig.  288)  of  the  con- 


SEC.  13]       MOTOR-DRIVE  POWER  REQUIREMENTS 


293 


nector.  There  is  no  electrical  path  directly  through  the  dummy- 
fuse-connector  because  the  old  "  blown"  fuse  from  which  it  was 
made  was  taken  apart  and  the  portions  of  the  fusible  conductor 
which  it  originally  contained  have  been  removed.  The  circuit 
of  the  motor  must,  therefore,  be  completed  through  the  watt- 
meter— or  an  ammeter  if  an  ammeter  is  used.  Where  a  test 
is  being  conducted,  as  indicated  in  Fig.  287,  a  portable  fuse 
is  often  inserted  in  the  circuit  which  contains  the  dummy-fuse- 
connector  so  that  the  motor  and  the  instruments  will  be  pro- 
tected against  over-load  while  the  test  is  being  made.  Such 
a  fuse  is  not  always  used,  but  to  insure  .against  accident 
should  be. 


Drill  Hole  Through, 
Ring£ 'Larger..    6 
than  Diameter  N" 
of  Machine 
Screw 


u, 


Equal  to  Diame  ter/ 
of  Fuse  Ferrule  ~' 

I-End  V.ev 


Nut  Soldered  on, 
/Make  Large 
'Enough  to 
Admit  Stove 
Bolt  Nut. 

Drill  and-  — 

Jap  for 

Voltmeter 

Screw  after 

Ring  is 

Soldered 

to  Fuse 


Equal  to  Width  of. 
Ferrule  M/nus/^'' 

H- Side  Viey 


--Hole  for  Wire 


,'This  Hole  Should 
be  Tinned inside 


FIG.  289.  —  Details  of  connecting 
strap. 


E-5ideYiew 


FIG.  290.  —  Lug  with  "open  hole" 
for  current  leads. 


445.  A  Dummy-fuse  -connector  for  61-  to  600-Ampere  N. 
E.  C.  Standard  Fuse  Blocks  is  indicated  in  general  construc- 
tion in  Fig.  288.  However,  this  illustration  is  dimensioned 
for  a  101-200-ampere  fuse.  Connecting  straps,  which  are  de- 
tailed in  Fig.  289,  are  soldered  to  the  ferrules  of  what  was  a 
fuse.  The  terminals  or  binding  posts  may  be  arranged  by 
soldering  on  to  each  of  the  connecting  straps  a  nut,  through 
which  a  brass  machine  screw,  B  and  B1,  turns.  Wattmeter 
or  ammeter  leads  may  be  connected  either  by  clamping  them 
under  the  heads  of  the  binding  post  (B  and  Bl)  or  they  may  be 
soldered  into  lugs  of  the  type  suggested  in  Fig.  290.  For  con- 
necting the  voltage  lead,  a  small  brass  machine  screw,  C,  is  ar- 
ranged to  turn  in  a  tapped  hole  in  one  of  the  connecting  straps. 


294 


ELECTRICAL  MACHINERY 


[ART.  446 


This  hole  should  be  drilled  or  tapped  after  the  strips  have 
been  soldered  to  the  ferrules.  The  other  voltage  lead  can  be 
connected  to  its  side  of  the  circuit  by  inserting  its  thin  metal 
terminal  lug  or  its  bared  end  between  the  fuse  knife  blade 
and  the  corresponding  contact  clip. 

446.  A  "Forked"  or  "Open  Hole"  Lug  of  this  type  is  desir- 
able in  that  it  may  be  clamped 
under  the  machine  screw  head  on 
the  connector  without  removing 
the  screw  entirely  from  its  hole. 
The  lug  (Fig.  290)  may  be  made 
by  filing  away  from  an  ordinary 
lug  the  portion  enclosed  in  the 
dotted  lines  in  the  illustration. 

447.  A  Dummy-fuse-con- 
nector for  Ferrule -contact  Fuse 
Blocks  may  be  arranged  as  detailed  in  Fig.  291.  A  brass  strip 
constituting  a  terminal  is  soldered  to  each  of  the  ferrules. 
Machine  screws,  provided  with  washers,  turning  in  tapped 
holes  in  these  strips  provide  for  the  connection  of  the  ammeter 
or  wattmeter  leads. 


'-Blown  or  Shot  fuse 


FIG.  291. — "Dummy-fuse" 
connector  for  a  small  capacity 
national-electrical-code-standard 
(10-30  and  30-60)  ferrule-con- 
tact fuse  block. 


from  Source-'* 


FIG.  292. — "Shot"  fuse  fitted  with  knife  switch  for  shunting  out  ammeter 

448.  A  Shunting-out  Switch  may  be  arranged  on  a  dummy- 
fuse-connector  as  diagramed  in  Fig.  292.  When  the  switch, 
S,  is  closed  the  ammeter  or  wattmeter  connected  across  the 
dummy-fuse-connector,  at  A  and  B,  is  shunted  out  of  circuit. 
As  indicated  in  the  illustration,  the  hinge  and  the  jaw  of  a 
knife  switch  are  respectively  soldered  to  each  of  the  ferrules 
of  the  "blown"  fuse.  The  lead  from  each  of  the  ferrules  to 


SEC.  13]       MOTOR-DRIVE  POWER  REQUIREMENTS 


295 


Pointer 


an  ammeter  or  wattmeter  may  be  soldered  to  the  ferrules  or 
connected  thereto  as  detailed  in  Fig.  288.  When  inserting  a 
dummy-fuse-connector  of  the  type  shown  in  Fig.  292  in  a  live 
circuit,  arrange  a  shunted  circuit  around  one  of  the  fuses  by 
placing  a  jumper  between  its  terminal  clips;  then,  pull  out  the 
fuse  and  insert  the  dummy-fuse-connector.  Then,  when  the 
switch,  S,  is  opened  the  current  taken  by  the  machine  under 
test  will  flow  through  the  ammeter  or  wattmeter.  Hence,  the 
operation  of  the  motor  continues  uninterruptedly  and  produc- 
tion is  not  blocked.  When  the  test  has  been  completed  the 
terminals  of  the  cut-out  are  again  shunted,  the  dummy-fuse- 
connector  withdrawn  and  the 
intact  fuse  replaced  before  the 
shunted  jumper  is  removed. 

449.  Graphic  Instruments 
are  Very  Desirable  for  Record- 
ing Motor  Power  Input  Tests. 
— A  graphic  wattmeter  of  one 
of  the  types  which  has  been 
satisfactory  for  work  of  this 
character  is  illustrated  in  Figs. 
293  and  294.  A  graphic  watt- 
meter will  record  on  a  strip  of 
paper  (Fig.  295)  a  graphic  record 
of  the  power  taken  by  the  motor 
driving  the  machine,  tool  or 
group  being  tested  at  the  dif- 
ferent instants  of  its  operation.  Such  a  curve  constitutes  a 
valuable  permanent  record  and  provides  information  which 
is  difficult  to  obtain  by  any  other  method.  Furthermore, 
where  a  curve-drawing  instrument  is  used,  it  can  be  con- 
nected in  the  test-motor  circuit  and  left  there  for  a  day  or 
several  days  or  for  a  month,  during  which  period  it  will  be 
automatically  recording  a  graph  of  the  performance  and  of  the 
power  taken  by  the  motor.  The  graph  indicates  clearly  just 
what  are  the  maximum  average  and  minimum  power  inputs 
to  the  motor  and  it  shows  the  time  relation  between  them. 
Where  a  company  is  purchasing  a  considerable  number  of 


Chart 


FIG.  293. — A  portable  graphic 
meter. 


296 


ELECTRICAL  MACHINERY 


[ART.  450 


motors,  a  graphic  instrument  will  usually  pay  for  itself  in  less 
than  a  year  by  enabling  its  owner  to  select  motors  of  the  small- 
est capacity  which  will  do  the  work. 


,-Back  Part  of  Case 


Dus  t-Proof  Cover  - 
Handle--., 


Chart' 


^-Writing  Mechanism 


FIG.  291. — Disassembled  view  of  an  Esterline  graphic  instrument. 


I-  The 
Steel  Shaft 


II -Graphic  Ammeter  Record 

FIG.  295. — Graphic-ammeter  record   indicating  the  power  required  to 
"rough  out"  the  steel  shaft  illustrated. 

450.  Graphic    Ammeters   are   sometimes   used   instead    of 
graphic  wattmeters  for  motor  test  work  and  on  direct-current 


SEC.  13]       MOTOR-DRIVE  POWER  REQUIREMENTS 


297 


circuits  where  the  voltage  regulation  is  reasonably  good  they 
may,  in  certain  cases,  be  preferred  to  graphic  wattmeters. 
The  reason  for  this  is  that  the  ammeter  is  simpler  than  the 
wattmeter  and  more  readily  connected  by  inexperienced  men 
than  is  the  wattmeter.  Such  a  direct-current,  graphic  am- 
meter will  draw  graphs  which,  by  taking  into  account  the 
voltage  (which  is  assumed  to  be  constant)  can  be  calibrated 
in  watts  or  horse-power.  However,  for  alternating-current 
work,  particularly  where  low  power-factors  are  encountered 
and  where,  therefore,  the  current  taken  by  a  motor  may  not  be 

Graphic 

.Wire  ~      Meter,  . 

1  Handle       n~ 


II- Rear  Elevation 


FIG.  296. — Portable  stand  for  graphic  wattmeter.      * 

at  all  proportional  of  the  actual  power  consumed,  a  wattmeter 
must  be  used. 

451.  One  Graphic  Instrument  Can  be  Used  to  Record  the 
Inputs  to  Motors  of  Various  Capacities  by  providing  suitable 
shunts  and  multipliers  for  the  direct-current  instruments  and 
series  and  shunt  transformers  for  the  alternating-current  in- 
struments.    It  frequently  occurs  that  the  electrical  manu- 
facturers do  not  regularly  list  these  "wide-range"  outfits  but 
will  usually  furnish  data  concerning  them  on  application. 

452.  A  Stand  for  a  Graphic  Instrument,  originally  designed 
for  switch-board  mounting,  can  be  constructed  as  shown  in 
Fig.  296.     Portable,  graphic  instruments  (Fig.  293)  are  regu- 


298 


ELECTRICAL  MACHINERY 


[ART.  453 


larly  manufactured  but  it  is  sometimes  necessary,  for  one  rea- 
son or  other,  to  use  those  of  the  switch-board  type.  The 
stand  of  Fig.  296  can  be  made  of  straight  grained  wood.  The 
actual  thickness  of  the  backboard  should  be  determined  by  the 
thickness  of  the  switch-board  panel  for  which  the  terminals  and 
supporting  studs  of  the  instrument  were  designed.  Fig.  297 
details  the  method  of  holding  the  nuts  for  the  leveling  screws. 
453.  An  Example  of  the  Record  of  a  Graphic  Ammeter* 
connected  in  a  machine-tool-drive  motor  circuit  is  represented 
in  Fig.  295.  This  graph  indicates  the  power  consumption  at 
different  instants  in  " roughing  out"  the  steel  shaft  shown  at 
Fig.  2957.  The  graph  reads  continuously  from  the  right  to 


j,- —Bore  Hole  & 
K" '  '*]  Larger  in  Diam. 
than  Diameter 
I         I  of  Bolt 


',    Bottom  Board' 
Hut  Serin  Depression' 


l-Section  A-A  n-Bottom  View 

FIG.  207. — Details  of  arrangement  of  leveling-screw  nut. 

the  left  in  accordance  with  the  sequence  of  the  time  intervals 
shown  along  the  lower  line  of  the  chart.  The  reference  letters 
on  the  graph  correspond  with  those  indicated  on  the  sectional 
view  of  the  shaft.  Hence,  although  the  shaft  was  reversed 
in  position  while  it  was  being  measured  the  power  and  time 
required  to  make  the  various  cuts  (shown  by  the  cross-hatched 
portion  in  7)  can  easily  be  ascertained. 

454.  To  Ascertain  the  Energy  Represented  by  a  Graphic 
Power  Record,  the  area  included  within  the  graph  of  the  record 
can  be  ascertained  with  a  planimeter.  This  area  will  be  pro- 
portional to  the  energy  (kwh)  consumed  during  the  period 
represented  by  the  graph. 

*  Westinghouse  Elec.  &  Manfg.  Co. 


SEC.  13]       MOTOR-DRIVE  POWER  REQUIREMENTS 


299 


T  Bar  of--* 
;     Iron  or 


455.  The  Practical  Determination  of  the  Torque  Required 
to  Drive  or  Start  a  Given  Load  may  be  effected  as  suggested  in 
Figs.  298  and  299.  With  the  method  of  Fig.  299  a  wooden 
clamp  is  bolted  on  the  belt 
driving  the  load  and  the  actual 
torque  measured  by  pulling  on 
a  spring  balance  fastened  to  the 
clamp.  The  force,  in  pounds, 
which  the  spring  balance  meas- 
ures will  be  the  force  required 
to  start  the  load  at  the  radius, 
L.  The  torque  is  obtained  by 
multiplying  this  force  by  the  the  torque  required  by  measuring 

^    c  Au     i  r       T     Jt  at  the  motor  coupling, 

length  of  the  lever  arm,  L.     In 

pulling  on  the  balance,  it  should  be  held  parallel  to  the  belt.  By 
applying  the  torque  formulas  given  in  Art.  237 — the  speed  and 
diameter  of  the  driven  pulley  being  known — the  horse-power  re- 


rCovpling 


Motor 


FIG.  .  298. — Ascertaining  by  test 


FIG.  299. — The  determination  with  a  spring  balance  of  the  torque  re- 
quired for  a  belt  drive. 

quired  by  the  drive  may  be  readily  computed.  Where  the 
driving  motor  transmits  its  power  to  the  load  through  a  coup- 
ling, the  scheme  suggested  in  Fig.  298  may  be  utilized.  The 
direction  of  the  pull  on  the  spring  balance  must  be  at  right 


300  ELECTRICAL  MACHINERY  [ART.  455 

angles  to  an  imaginary  line  passing  through  the  center  of  the 
shaft  and  through  the  point  where  the  balance  is  attached  to 
the  testing  lever.  Where  a  machine  is  gear-driven,  the  torque 
may  be  ascertained  by  placing  the  hook  of  a  spring  balance 
over  one  of  the  arms  of  the  gear  and  pulling  thereon,  noting 
the  pounds  force  required  to  start  and  to  move  the  load. 


SECTION  14 
MOTOR  GENERATORS  AND  FREQUENCY  CHANGES 

456.  A  Motor-generator  Set*  is  Defined  thus:  "A  trans- 
forming device  consisting  of  a  motor  mechanically  coupled  to 
one  or  more  generators/7 

457.  A  Motor-generator  Set  Comprises  two  electrically  dis- 
tinct elements,  a  motor  and  a  generator  coupled  mechanically 
together  and  usually  mounted  on  the  same  bed  plate.     The 
motor  and  the  generator  may  each  be  of  any  commercial  type. 
See  Fig.  300.     A  motor-generator  converts  electrical  energy 


FIG.  300.— A  "Ridgway"  motor-generator  set.  The  210-h.p.,  2200- 
volt,  3-phase,  60-cycle,  induction  motor  drives  two  70-kw.,  125-volt, 
700-r.p.m.  direct-current  generators. 

from  one  voltage  or  frequency  to  some  other  voltage  or  fre- 
quency or  from  alternating  to  direct  current.  Where  the 
motor  is  operated  by  alternating  current,  a  synchronous  motor 
is  frequently  used  because  of  its  ability  to  correct  low  power- 
factor,  but  an  induction  motor  may  be  used.  The  motor  or 
generator  may  be  either  alternating  current  or  direct  current. 
Figs.  Ill,  112  and  113  show  a  motor-generator  mine-hoist 
outfit;  refer  to  the  text  accompanying  these  illustrations  for 
information  relative  to  this  application. 

*  STANDARDIZATION  RULE  104,  A.  I.  E.  E.,  June  28,  1916. 

301 


302 


ELECTRICAL  MACHINERY 


[ART.  458 


458.  Alternating-current  Motor,  Direct-current  Generator 
Sets  (Fig.  301)  are  used  to  obtain  a  direct  e.m.f.  where  the 
source  of  energy  is  an  alternating  one.     The  alternating-cur- 
rent motor  may  be  of  either  the  induction  or  the  synchronous 
type,  each  of  which  has  its  advantages  for  certain  applications, 
as  suggested  below. 

459.  An  Induction-motor  Drive  for  a  Motor-generator  Set 
has  in  its  favor  the  features  of  low  first  cost  and  simplicity. 
Induction  motors  may  be  wound  for  high  pressures — as  great 
as  13,000  volts — which,  in  many  cases,  will  render  the  use  of 
transformers  unnecessary.     Because  of  the  fact  that  induction 
motors  have  a  lower  speed  at  full-load  than  at  no-load  (Art. 
316),  where  it  is  desired  that  the  direct-current  generator 
driven  by  such  a  motor  maintain  a  constant  voltage,  the  gen- 


-  Source  of 
t  Energy 


,2200 -Volt 
'^  Three-Phase 
60-Cycle 
Alternating- 


A.C. 

Motor 


1 


50-Vo/t 
Direct-  Current 


"Commutator 


FIG.  301. — Illustrating  the  application  of  a  motor  generator. 

erator  must  be  suitably  over-compounded  to  maintain  close 
voltage  regulation. 

460.  A  Synchronous-motor  Drive  for  a  Motor-generator  Set 
is  probably  the  more  frequently  used,  particularly  for  sets  of 
considerable  output.  An  important  advantage  of  the  syn- 
chronous-motor drive  is  that  an  over-excited  synchronous 
motor  acts  as  a  synchronous  condenser  (Art.  342)  and  hence 
may  be  utilized  in  improving  the  power-factor  of  the  supplying 
system.  Due  to  the  fact  that  the  synchronous  motor  (Art. 
342)  is  inherently  a  constant-speed  motor,  where  a  direct- 
current  generator  is  driven  by  one,  the  voltage  on  the  direct- 
current  circuit  can  be  maintained  practically  constant  at  all 
loads  regardless  of  the  alternating-current  voltage  and  of  the 
distance  of  the  motor-generator  set  from  the  energy-supplying 
station. 


SEC.  14]     MOTOR  GENERATORS,  FREQUENCY  CHANGES    303 

461.  A  Frequency  Changer  motor  generator  set  is  one  con- 
sisting of  an  alternating-current  motor  and  an  alternating- 
current  generator  each  of  a  different  frequency.  For  illustra- 
tion (Fig.  302)  a  25-cycle  motor  may  drive  a  60-cycle  genera- 
tor. The  motor  must  be  of  the  synchronous  type  if  it  is  nec- 
essary that  the  frequency  of  the  supplied  circuit  be  maintained 
constant  because  (Art.  316)  the  speed  of  an  induction  motor 
decreases  with  the  load.  Frequency  changers  are  used  when 
the  frequency  of  the  supplied  circuit  must  be  different  from 
that  of  the  supply  circuit.  In  other  words,  they  must  be  in- 
stalled where  it  is  necessary  to  interchange  power  between  two 
circuits  or  systems  of  different  frequencies. 


^•'Supplying  System  25  Cycles 

Receiving  System  60  Cycles-* 

\ 

1 

^--Jhree- 
''  Phase 
Alternating- 
Current 
Circuit 

II 

h 

t:' 

/^ 

1 

«M 

L 

I 

^i 

-: 

4 

it 

-7^ 

^v 

^ 

25- 
Cyde 
Motor 

M 

60- 
Cycle 
Generator 

> 

k^ 

V£ 

..:.M..:. 

ri^^M^-M 

i/ 

^i 

....   6... 

u^i^v«£^ 

iiX 

^ 

FIG.  302. — Illustrating  the  application  of  a  frequency-changer,  motor 

generator. 

EXAMPLE. — Electrical  energy  is  developed  at  the  Keokuk  hydroelec- 
tric plant  at  25  cycles  and  transmitted  at  that  frequency  to  St.  Louis  and 
to  other  localities.  However,  inasmuch  as  a  frequency  of  60  cycles  is,  in 
general,  more  desirable  for  lighting — because  the  "flicker"  of  incandes- 
cent lamps  which  occurs  with  25  cycles  is  not  visible  with  60  cycles — 
and  furthermore,  since  60-cycle  motors  of  given  outputs  and  speeds  cost 
less  than  do  equivalent  25-cycle  motors,  a  frequency  of  60  cycles  is  used 
in  St.  Louis  and  in  most  of  the  other  communities  supplied  from  this 
plant.  Hence,  at  each  of  these  localities  where  60-cycle  energy  is  utilized, 
a  frequency  changer — a  25-cycle  synchronous  motor  driving  a  60-cycle 
generator — has  been  installed. 

462.  The  Two  Machines  of  a  Frequency  Changer  are  Me- 
chanically Coupled  Together  hence  the  speed  of  both  machines 
must  be  one  which  will,  considering  the  number  of  poles  of  each, 


304 


ELECTRICAL  MACHINERY 


[ART.  462 


provide  the  correct  frequency.  Where  the  ratio  of  the  two 
frequencies  is  an  even  number,  the  result  is  readily  obtained. 
For  instance,  for  converting  from  25  to  50  cycles  the  generator 
should  have  twice  as  many  poles  as  the  motor.  Often  the 
frequencies  involved  are  25  and  60.  The  following  tables 
show  some  synchronous  speeds  for  25-  and  60-cycle  machines. 


Number  of  poles  . 

Synchronous  speed 

25  cycles 

60  cycles 

2  poles               

1,500 
750 

500 
375 
300 
214.27  + 
166.6  + 
150 
125 

3,600 
1,800 

1,200 
900 
720 
514 
400 
360 
300 

4  poles                         .          

6  poles     

8  poles                         

10  poles                                      

14  poles                

18  poles                                  

20  poles                                               •    •  •  • 

24  poles                .          

NOTE. — From  the  table  it  is  evident  that  the  only  pole  combinations 
that  give  precisely  the  same  speed  are  10  poles  for  25  cycles  and  24 
poles  for  60  cycles.  Each  gives  300  r.p.m.  This  is  a  low  speed  and  the 
cost  of  a  set  of  such  a  speed  is  relatively  high.  For  practical  purposes  a 
frequency  of  62^  cycles  can  often  b'e  used  instead  of  60.  A  four-pole, 
750-r.p.m.,  25-cycle  motor  driving  a  ten-pole  generator  will  provide 
cycles. 


INDEX 


Adjustable-speed  motors,  d.c.,  classifica- 
tion       47 

reliance 42 

Air   gap  of  d.c.  generators,  variation 

in  flux  distribution  in,  illustration. .      17 
Alternating-current    generators,    see 

Generators,  a.c. 
motors,  see  Motors,  a.c. 
Alternators,  see  Generators,  a.c. 
Ambient  temperature,  a.c.  generator 

guarantees 166 

definition 30,  166 

d.c.  generator  guarantees 28 

motor  guarantees 29 

Ammeter,  for  compound,  d.c.  genera- 
tor       56 

graphic,  example  of  record 298 

used  instead  of  wattmeters.  .  .    296 
knife  switch  for  shunting  out.  .  .    294 
shunt,  mounting  on  generator. . .      58 
Arc-light  (constant-current)  d.c.,  gen- 
erator regulated  by  field  variation. .        5 
Armature,  a.c.  generator,  three-phase, 

methods  of  connecting 168 

two-phase,  methods  of  con- 
necting         168 

or  stator  turbo  generator,  illus- 
tration     161 

three-phase     generator,     con- 
necting, diagram 170 

circuit,   open,  sparking  may  be 

due  to 128 

circuit,  see  Circuit,  armature. 

coil,  d.c.,  grounded,  test 138 

hot,  cause 140 

reversed,  test 139 

connection,  d.c.,  shunted,  series 

motor 77 

control,  d.c.,  objections 94 

regulator,  d.c.,  construction...     93 

speed  regulators,  d.c 93 

operation,  d.c 94 

d.c.,  balancing. 147 

rack  for 147 

series-wound,  number  of  brush 

sets  in 15 

to  insure  proper  balance 146 

tested  for  common  troubles.  .130 
handling,  67 

heating,  causes 140 

short-circuits,  test 138 

stand,  description 147 

synchronous     motor,     improper 

connections 238 

testing  where  only  a.c.  or  low- 
voltage  cells  are  available.  ...    132 

testing,  with  high-voltage  a.c 132 

unbalance,  noise  caused  by 146 

windings,    d.c.,    flying    grounds, 
short-circuits    and    open    cir- 
cuits     140 

Automatic    regulation    of   d.c.,   con- 
stant-current generator 3 

starter,  see  Starter,  automatic. 


Auto-starters,  to  limit  starting  cur- 
rent, synchronous  motors,  a.c 232 

Auto  transformer  for  starting  induc- 
tion motors,  see  Compensator. 

B 

Balking,  a.c.,  induction  motors 269 

Ball  bearings,  examples 144 

vertical  motors 146 

Bar  and  coil  leads,  poor  connection 

between 137 

Bearing,  bearings. 

hot,  correction 144 

troubles,  motors  and  generators.    143 
a.c.  generators  and  motors.  .  .   263 

induction  motor 273 

motors 265 

synchronous  a.c.  motors 238 

warm,  causes 144 

a.c.,  induction  motor,  oil  leakage.  273 

cleanliness  essential 144 

motor,    grooves    to    prevent    oil 

leakage 274 

Bipolar  machine,  definition 15 

Blackening,  commutator 116 

Blow  holes  in  frame  castings  some- 
times cause  sparking 126 

Brake,  prony,  different  forms 149 

motor     horsepower     determined 

.     with • 149 

torque  concept  basis  for 149 

Braking,  dynamic,  connections 98 

d.c.  motors,  method: 97 

heating 97 

principal  advantages 97 

when  used 97 

Brush,  carbon,  pressure 124 

contact  resistance 124 

fine    grain,    soft;    possibility    of 

using  with  slotted  commutator  121 
position     of     commutating-pole 

machine 20 

sets  in  d.c.  machines,  number  of.      15 
shifting,  regulation  of  constant- 
current  generator  by 4 

trouble,  glowing  and  pitting 114 

Brushes,  adjustment  and  care 122 

carbon,  glowing  and  pitting 126 

chattering 127 

copper,  reversal  of  current  must 

be  accurately  effected 113 

d.c.  generators,  management. ...      66 

effect  of  shifting  on  speed 41 

should  be  fitted  to  commutator .  .    123 

sparking,  causes 125 

Building  up,  shunt  d.c.  generator. ...        7 
Buses,  equalizer,  required  for  three- 
wire  machines 58 


Capacity,     overload,     commutating- 
pole  motors 34 

normally  rated  generators 33 

steam  engines ,  33 


20 


305 


306 


INDEX 


Carbon  brush,  see  Brush  carbon. 
Cascade,    speed    control,    a.c.,    poly- 
phase, induction  motors 257 

Characteristics,  principles  and  con- 
struction, induction  and  repul- 
sion a.c.  motors 190 

speed,     compound-wound,     d.c. 

motors 44 

d.  c.  motors,  series,  shunt  and 

compound 34 

shunt  motor 40-42 

series  and  compound-wound 
motors,  graphic  compari- 

sion 46 

speed-torque,  series  d.c.  motor. .      37 

Chattering  of  brushes 127 

Circuit,     armature,     open,     sparking 

due  to 128 

symptoms  of  trouble  due  to.  ...    128 
Circuit-breaker,    polyphase    a.c.    in- 
duction motor,  protection. .        236 
vs.  fuses,  advantages  and  dis- 
advantages      76 

Circuit,  field,  open,  locating 142 

open  armature,  tests 137 

three-phase,  phasing  out 180 

Classification,  single-phase  a.c.   mo- 
tors    206 

Coil,  armature,  grounded,  test 138 

hot,  causes 140 

open-circuited,  test 137 

reversed,  test 139 

field,  grounded,  low  located 140 

heating  causes 142 

open-circuited,  locating 141 

inducing,     a.c.,     for     localizing 

armature  troubles 133 

leads,  crossed,  test 139 

Coils,  balance  of  three-wire  genera- 
tor, connecting  requirements 

for 26 

series,     three-wire,     compound- 
wound  generator,  division ....     25 
short-circuited,    a.c.    motor    or 

generator,  to  locate 274 

Collector-ring   troubles,   a.c.,   induc- 
tion motor 271 

Commutating  pole    d.c.,     generator, 
correct  polarity  at  full-load 

and  incorrect  at  no-load 66 

shunt  motor,  speed  regulation.     41 
generators  and  motors,  see  Gen- 
erators,   commutating    pole    or 
motors,  commutating  pole. 
machines,   see  Motors  or  genera- 
tors, commutating  pole. 
poles,  effect  on  speed  regulation, 

d.c.  motors 46 

object 17 

winding  of 20 

three-wire   generator    connec- 
tions       27 

windings,  determining  polari- 
ties      66 

Commutation,  d.c.  motors,  deter- 
mines overload  capacity 34 

process 113 

Commutator,  all  brushes  should  be 

fitted 123 

bar,  loose,  sparking 118 

blackening 116 

commutators 115 

heating 115 

high  mica 121 

high-mica,  roughened,  to  remedy  122 
hot .    115 


Commutator,  loose,  to  correct 119 

management 67 

methods  of  slotting 121 

reason  for  slotting 120 

roughness,  correcting lie 

due  to  loose  bar us 

rough,  sparking 1 14 

sandpaper  to  smooth 118 

segments,  loose 115 

slotted,  care 121 

possibility  of  using  fine  grain, 

soft  brush 121 

slotting 119 

to  smooth  with  grindstone 117 

trouble,  glowing  and  pitting 114 

truing  with  file 118 

what  is  accomplished  by  slotting.  120 

Compensated  d.c.   generator 21 

conductors  for 16 

induction  a.c.  motor,  see  Motor, 

a.c.,  compensated  induction. 
Compensating  winding,  a.c.  motor...  213 
Compensator,   a.c.,  induction  mrtor 

connected  to,  will  not  start. . .  271 
for    starting    squirrel-cage    a.c. 

motors 245 

method  of  starting  several  poly- 
phase a.c.,  induction  motors 

from  one 250 

starter,  starting  currents  and 
starting  torques  of  squirrel- 
cage  induction  a.c.  motors, 
with  different  impressed  vol- 
tages by  using 246 

starters,  fuses  used  with 251 

high-voltage  motors,  a.c.,  with 

no-voltage  release 248 

starting,  overload  release  coils, 

arrangement 248 

for  a.c.  motors,  high  voltage 

large  capacity 247 

no-voltage  release  can  be  pro- 
vided..    250 

for     squirrel-cage     induction 

a.c.  motors 243 

induction    motors    with    and 

without 244 

several  motors  from  one 250 

taps 246 

to    limit   starting    current,    a.c. 

synchronous  motors 232 

troubles,  a.c.,  induction  motor. .   270 
when   used   starting   torque   re- 
duced  to   value   required   by 

load 245 

Compounding   d.c.   generator,   effect 

on  voltage 154 

over,  voltage  increase  due  to ...      13 
Compound    dynamo,    see    Generator, 

compound. 
Compound    motor,  see    Motors,  d.c., 

compound  wound. 

Compound-wound  d.c.  generators,  see 
Generators,  d.c.,  compound  wound. 

Condensers  a.c.,  synchronous 229 

definition 231 

Conductors    for    d.c.,     compensated 

generators 16 

Connecting    leads,     d.c.,     compound 

generators 56 

Connection,  field-spool,  reversed 142 

d.c.,  shunt  generators  for  parallel 

operation 51 

motors,  illustration 39 

switchboard,  synchronous,  a.c. 
motor 232 


INDEX 


307 


Constant-current  d.c.  generators,  see 

Generators  d.c.,  constant  current. 
Construction     of     commutating-pole 

d.c.  generators 18 

principles  and  characteristics,  in- 
duction    and     repulsion     a.c. 

motors 190 

Contact,  brush,  resistance. 124 

Continuous  rating,  definition 29 

Control,  armature,  objections 94 

regulator,  construction 93 

speed  regulators,  operation ...      94 
d.c.  motors  with  flywheel  motor- 
generator 99 

definition 1 

equipment,  motor,  a.c.,  National 
Electrical     Code     installation 

requirements 239 

float,    automatic    starters,    a.c. 

pump  motors,  connecting. .   261 
switch,  a.c. .single-phase  motor.  261 
speed,  a.c.  polyphase,  induction 
motor,   by  adjusting 

frequency 258 

by    adjusting    primary 

voltage 254 

by     changing     number 

of  poles 256 

of  secondary  phases.   261 
with  double  primary. . .   255 

motors 252 

by  adjusting  resistance 
of  secondary  circuit.   252 

cascade 257 

compound  d.c.  motor 78 

primary,  a.c.,  polyphase  in- 
duction motor 255 

secondary,  a.c.  motor 253 

shunt  d.c.  motors. ".     77 

Controller,  drum,  rotary  or  machine- 
tool  type,  d.c.  motors 78 

type,  advantages 80 

operation 80 

magnet-switch,  d.c.  motors 85 

regulating,  connections,  com- 
pound-wound d.c.  motor. . .  83 

d.c.  series  motor 82 

rheostatic 72 

Controllers,   crane,   d.c.   motors,   ar- 
rangement      95 

Controlling,  devices  for  a.c.  motors . .   239 
Copper  brushes,  reversal  of  current 

must  be  accurately  effected 113 

Crane    controllers,    d.c.    motors,    ar- 
rangement       95 

Cumulative-compound   windings  for 

d.c.  motors  and  generators. ...      14 
-wound  d.c.  motors,  description.     45 
Current,    computation,    single-phase 

a.c.  motor 226 

two-phase  motor  a.c 205 

cross,    a.c.    generators,    parallel 

operation 184 

d.c.  motor,  to  compute 48 

or  voltage,  a.c.,  single-phase  gen- 
erator, kilovolt-amperes  out- 
put, computation 174 

three-phase  generator,  kilovolt 
amperes  output,  computa- 
tion.  ..... 174 

reversal     must     be     accurately 
effected,  with  copper  brushes. .   113 

starting,  a.c.  motors 200 

squirrel-cage  inductions  a.c.  mo- 
tors.withdifferentimpressed  vol- 
tages using  compensator  starter.  246 


Current,  voltage,  efficiency,  power- 
factor,  a.c.  generator,  horse- 
power required  to  drive, 

computation.'. 172 

three-phase  generator 176-204 

Curves,  see  Graphs. 


Data,  performance,  d.c.  motors 34 

Delta-star  method  of  starting  three- 
phase,  squirrel-cage  induction  a.c. 

motors 251 

Detector,    telephone    receiver,    used 

with  inducer 135 

Differential     compound-wound     d.c. 

motors,  description. 45 

-compound     windings     for     d.c. 

motors  and  generators 14 

Direct-current   generators,   see  Gen- 
erators, d.c. 

power  and  lighting,  compound- 
wound    d.c.,   generators    used 

for 10 

Division,  load,  compound  generators.      13 

Drum  controllers 80 

rotary  or  machine-tool  type, 

d.c.  motors 78 

Drying  out  motors  and  generators. . .      68 

Dummy-fuse  connectors 292 

Dynamic   braking,   see   Braking  dy- 
namic. 

Dynamo,  definition 1 

Dynamos,  see  Generators. 


Efficiency,  computation,  single-phase 

a.c.  motor 226 

two-phase  motor,  .a.c 205 

d.c.  generator,  average  value 32 

motor,  to  compute 48 

computation      of       three-phase 

motor 204 

loss   with   a.c.   induction   motor 

at  reduced  speeds 195 

or  power  factor,  kilowatt-ampere 
output,  horse-power  required, 
a.o.,  three-phase  generator, 

computation 175 

power  factor,  voltage  or  current, 
a.c.,  three-phase  generator, 
horse-power  required,  com- 
putation   176 

voltage,   current,   power  factor, 
a.c.     generator,     horse-power 
required  to  drive,  computation  172 
Electric  motor,  see  Motor. 
Electromotive  force,   a.c.   generator, 

how  generated 164 

Energy     ascertained     from     graphic 

power  record  with  a  planimeter 298 

Engine,   size  required  to  drive   a.c. 

generator 177 

d.c.  generator 32 

steam,  overload  capacity 33 

Equalizer  buses,  required  for  three- 
wire  machines 58 

or     equalizer     connection,     d.c. 

generator 55 

Esterline  graphic  instrument 296 

Excitation  of  generator  fields 2 

test,  a.c.  generator 276 

Exciter,     belt-driven,     for     vertical 

water-wheel  generator 171 


308 


INDEX 


Exciter,     belt-driven,    capacity     re- 
quired examples 171 

should  be  ample 170 

drives 170 

Exciters,   a.c.   generators,   character- 
istics     169 

separately-driven,  preferable 169 

Exploring    terminal    and    test    lamp, 
convenient  arrangement 136 


Field,  a.c.,  turbo  generator 161 

circuit,  open,  locating 142 

coil,  d.c.  generators  or  motors, 

placing 65 

grounded,  how  located 140 

heating  causes 142 

open-circuited,  locating. .....    141 

d.c.    generator,    to    excite    from 

outside  source 60 

-discharge  switches  and  resistors .    102 
for  automatically  discharging 
field  circuits,  a.c.  generators.   171 

flux,  d.c.  motor,  test 142 

generator,  excitation  of 2 

magnetic,  a.c.  induction  motor. .    191 

polarity,  testing 62 

relay  switches 88 

rheostat,  compound-wound  d.c., 

generators 10 

shunting,  series  d.c.  motor 77 

-spool  connection,  reversed 142 

series,   equalizer   buses  required 

for  three-wire  machines 58 

troubles,  a.c.  synchronous  motor.  235 
variation,    arc-light,    d.c.    (con- 
stant-current) generator  regu- 

>ted  by & 

windings  on  d.c.  generator  frames, 

direction  of,  illustration 15 

File  for  tuning  commutator 117 

Flashing,  commutator,  sparking 130 

Flat-compounded    compound-wound 

d.c.  generator,  definition 9 

Float-control  automatic  starters,  a.c. 

pump  motors,  connecting 261 

switch. 101 

control,  a.c.  single-phase  motor.  261 
Flux,  action  of  in  commutating-pole 

d.c.  generator 21 

distribution  in  air  gap  of  d.c. 
generator,  variation,  illustra- 
tion of 17 

field,  d.c.  motor,  test 142 

Flywheel     motor-generator     control 

of  d.c.  motors 99 

function 101 

Frames,  d.c.  generator,  direction  of 

field  windings  on,  illustration 15 

Frequencies    a.c.,    polyphase    induc- 
tion motor,  performance  graph  259 
and  voltages,  different,  a.c.,  poly- 
phase  induction    motor,    per- 
formance graph 259 

a.c.   motor   operation,   effect   of 

changes 196 

changer,  motor  generators 301 

motor  generator,  construction.    303 

ratio  of  frequencies 304 

speed  and  number  of  poles,  a.c. 
generator,   relation  be- 
tween, formulas 164 

synchronous    induction 
motor. . .  201 


Fuse,  dummy,  connectors 292 

used  with  compensator  starters.    251 
vs.    circuit-breaker,    advantages 
and  disadvantages 76 


General  Electric  Co.,  repulsion  induc- 
tion motor 217 

Generator,  .  generators,     alternating 
current. 

bearing  troubles 263 

characteristics  of  exciters 169 

different  types 157 

direct-connected,  engine-type, 

illustration 159 

electromotive  force,  how  gen- 
erated   164 

excitation     or     magnetization 

test 276 

exciters  compound- wound. . . .    169 
field    discharge    switches    and 
resistors    for    automatically 
discharging  field  circuits...    171 
general  construction  and  defi- 
nition of  parts 157 

horse-power  required  to  drive, 
voltage,  current,  efficiency, 
power  factor,  computation.  172 

how  rated 166 

hunting 186 

prevents  parallel  operation.    182 

inductor,  definition 157 

insulation  resistance  test 277 

kilowatt  output,  voltage  cur- 
rent or  power  factor,  com- 
putation   176 

locating  short-circuited  coil. .  .   274 

management 178 

maximum  temperature  rise.  .  .    166 
operating  in  parallel,  switch- 
board diagram 184 

parallel  operation,  adjustment 

of  field  current 183 

cross  current 184 

division  of  load 182 

performance  guarantees 166 

principles,     construction     and 

characteristics 157 

relation  between  speed,  fre- 
quency and  number  of  poles, 

formulas 164 

revolving-armature,  definition.  157 

illustration 159 

revolving-field,  definition 157 

short-circuited  armature  coils, 

"inducer"  for  locating 274 

single-phase,  current  or  vol- 
tage, kilovolt-amperes  out- 
put, computation 174 

elements 158 

horse-power  required  to 
drive  based  on  kilovolt- 
ampere  output,  computa- 
tion   172 

kilowatt  output,  computa- 
tion   173 

principle 165 

six-phase,  grouping  of  coils. .  167 
size  engine  required  to  drive.  177 
steam-turbine,  illustration...  160 
successful  parallel  operation 

requirements 182 

surging 186 

synchronizing  requirements. ..  178 
synchronous-impedance  test.  .  276 


INDEX 


309 


Generator,    generators,     alternating 

currnet,  testing 276 

three-phase 167 

armature,  methods  of  con- 
necting  . .  168  , 

connecting  armature,  dia- 
gram   170 

current  or  voltage,  kilovolt- 
amperes  output,  compu- 
tation    174 

horse-power  required  to 
drive,  kilowatt-ampere 
output,  power  factor  or 
efficiency,,  computation..  175 
horse-power  required,  vol- 
tage, current  efficiency  or 
power  factor,  computa- 
tion   176 

large,  temperature  test 278 

load  test 277 

synchronizing  connection  for 

more  than  two 180 

temperature  test 279 

to  start  a  single 188 

to  run  in  parallel 188 

troubles 263 

turbo,  construction 160 

illustration 160 

rotor  or  field 161 

speeds 162 

stator  or  armature,  illus- 
tration   161 

ventilation 162 

two-phase,  armature,  method 

of  connecting. 168 

diagram 167 

principle 166 

vertical       waterwheel,       belt- 
driven  exciter  for ....    171 

construction 163 

which  is  running  in  parallel, 

to  cut  out 188 

Y-connected,  diagram 169 

bearing  troubles 143 

classification  of,  by  poles 16 

definition 1 

direct  current. 

armature,  handling 68 

bipolar,  definition 15 

commutating-pole,    action    of 

flux  in 21 

and  compensated,  compari- 
son   22 

construction   and  operation 

of 18 

correct  polarity  at  full-load, 
incorrect  polarity  at  light 

load 66 

operation  in  multiple 59 

principal  advantage  of 16 

to  reverse  direction  of  rota- 
tion   60 

commutators,  management.. .      67 

compensated 21 

conductors  for 16 

compound 8,  9 

ammeters  for 56 

commutating  pole,  perform- 
ance data 44 

connecting  leads  for 56 

determination    of    external 

characteristic 153 

directions  for  starting 51 

effect  on  voltage 154 

field  rheostat 10 

for  power  and  lighting 10 


Generator,    generators,    direct    cur- 
rent,   compound,    getting 

to  "pick  up" 61 

magnetization  graph  deter- 
mines voltage  regulation.  13 

parallel  operation  of 53 

series  shunt  for  components 

of 15 

shutting  down  when  operat- 
ing in  parallel 52 

to   adjust   division    of   load 

between  two 56 

constant-current 2 

arc-light,  regulated  by  field 

variation 5 

automatic  regulation 3 

regulation     of,     by     brush 

shifting 4 

definition 2 

differential    and     cumulative.- 

compound  windings 14 

drying  out 68 

efficiency  average  value 32 

equalizer    or    equalizer    con- 
nection    55 

failure  to  excite 62 

when  starting 60 

fields,  excitation  of 2 

flat-compounded      compound- 
wound,  definition 9 

frame     castings,     blow     holes 

sometimes  cause  sparking  126 
•     direction   of    field    windings 

on,  illustration 15 

how   to   reverse    direction    of 

rotation 59 

long-shunt 11 

magnetization-graph  test,  how 

conducted 151 

management 50 

brushes 66 

motor  generator,  a.c.  motor. . .  302 

multipolar,  definition. 15 

number  of  brush  sets  in 15 

normally  rated,  usual  overload 

capacity 33 

number  of  poles 15 

operating  at  different  speeds, 

voltage  regulation 12 

below  normal  speed 14 

operation  of  shunt  and  com- 
pound in  parallel 57 

over-compounded 9 

performance  guarantee 28 

placing  field  coils 65 

poor  connection  between  com- 
mutator bar  and  coil  leads .  .  137 

separately-exf  ited •     4 

series,  application .  .  . 2 

shunt  on,  form  of 15 

wound 2 

short-shunt  compound-wound.  11 

shunt 6 

exciting  current 7 

external  characteristic  test.  154 

operation  at  constant  speed,  7 

parallel  operation 50 

connections 51 

-wound,  directions  for  shut- 
ting down 50 

wound   for  starting 50 

voltage  of 8 

size  engine  required ! 

sources  of  losses 27 

temperature    of,     effected    by 

speed  changes 14 


310 


INDEX 


Generator,  generators,  direct  current, 

test,  loading  back  method.  .    152 
terminal    voltage,     increase 
due  to  over-compounding.. .      13 

testing 148 

for  polarity 54 

three- wire,  ammeter  shunt  on.     58 
compound-wound,     division 

of  series  coils 25 

connecting  balance  coils,  re- 
quirements of 26 

connections,    commutating- 

pole 27 

definition 22 

equalizer  buses  required  for .     58 

operation  in  multiple 57 

starting  and  shutting  down, 

method 53 

switchboard  connections 58 

time   required    for   building 

up 7 

to  compute  horse- power  input, 

kilowatt  output,  efficiency     30 
kilowatt  output,  current,  or 

voltage ^ 30 

troubles 105-112 

two-wire,  connections  to  feed 

three-wire  system 58 

variation  in  flux  distribution 

in  air  gap  of,  illustration  of . .     17 
when   refuses   to   excite,   pro- 
cedure      61 

windings,  danger  of  over-heat- 
ing when  drying 68 

frequency-changer 301 

insulation    resistance,    measure- 
ment    155 

performance  specifications  for.  .  .        1 
Glowing  and  pitting  of  carbon  brushes  126 

brush,  causes 126 

Graph,  definition 4 

motor,      a.c.     induction,     char- 
acteristic      198 

performance,    d.c.    series-wound 

method  of  reading 38 

a.c.,  polyphase  induction  mo- 
tor, with  different  frequen- 
cies and  voltages 259 

d.c.  series  motor 36 

speed-torque,    secondary   speed- 
control  a.c.,  induction  motor. .   253 
Graphic  ammeter,  example  of  record.  298 
instrument,  one,  can  be  used  to 
record  inputs  to  motors  of 

various  capacities 297 

for  recording  motor  power  in- 
put tests 295 

.     portable  stand  for 297 

power  record,  energy  ascertained 

with  a  planimeter 298 

Grindstone,  to  smooth  commutator. .    117 

Grounded  armature  coil,  test 138 

field  coil,  how  located 140 

Grounds,    flying,    in    d.c.    armature 

windings 140 

Guarantees,  performances,  a.c.  gen- 
erators    166 

motors 197 

d.c.  generators 28 

temperature  rise,  a.c.  motors 197 


Heating,  armature,  causes 140 

commutator 115 

field  coils,  causes 142 

dynamic  braking 97 


Horse-power,     computation,     single- 
phase  a.c.  motor 226 

current,  voltage,  power-factor, 
efficiency  three-phase  motor.  204 

two-phase  motor  a.c 205 

formulas,  motors 148 

of  d.c.  motors,  effect  of  torque 

and  speed 34 

output  a.c.  motors,  test  to  deter- 
mine   278 

d.c.  motor,  to  compute 48 

required,  a.c.,  three-phase  gen- 
erator, voltage,  current, 
efficiency  or  power  factor, 

computation 176 

of  motor,  often  over-estimated.  288 
to  drive  a.o.  generator,  voltage, 
current,  efficiency,  power 

factor,  computation 172 

single-phase  generator  based 
on  kilovolt-ampere  out- 
put, computation 172 

three-phase  generator,  kilo- 
watt-ampere output, 
power  factor  or  efficiency, 

computation 175 

Hot  box,  causes 144 

Hunting,  a.c.  generators 186 

induction  motors 272 

prevention 187 

prevents  parallel  operation 182 

synchronous,  a.c.  motors 237 


Inducer  for  locating  short-circuited 
a.c.  motor  or  generator  arma- 
ture coils 274 

principle 133 

telephone  receiver  detector  used 

with. 135 

Inducing  coil,  a.c.  for  localizing  arma- 
ture troubles 133 

Induction  motors,  see  Motors,  A.C., 

Induction. 
Input,     a.c.,     single-phase     motor, 

measuring 279 

to  motors  of  various  capacities, 
one  graphic  instrument  can  be 

used  to  record 297 

test,  a.c.,  three-phase  motor, 
"two- watt  meter"  accu- 
rate method 281 

"one- wattmeter "  method.  .   283 
"  one-wattmeter-and-Y-box  " 

method 284 

polyphase-wattmeter  meth- 
od.     282 

method  where  neutral  motor 

is  brought  out 285 

motor  power,  graphic  instru- 
ments for  recording 295 

Instruments,   graphic,   for  recording 

motor  power  input  tests.  . . .   295 

portable  stand  for 297 

Insulation  resistance,  see  Resistance, 

Insulation. 
Interpoles,  see  "  Commutating  Poles." 

K 

Kilowatt    input,  d.c.  motor,  to  com- 
pute.       48 

Kimble  single-phase  a.c.  motors 221 


Lamps,  synchronizing 181 


INDEX 


311 


Leads,  bar  and  coil,  poor  connection 

between 137 

coil,  crossed,  test 139 

Lighting    compound- wound    genera- 
tors used  for 10 

Load,  adjustable,  d.c.  compound  gen- 
erator, determination  of  ex- 
ternal characteristic 153 

division,  compound  generators. .      13 
to   adjust   between    two   d.c., 

compound- wound  generators.     56 
power  required  to  drive,  to  deter- 
mine by  test 289 

test,  d.c.  shunt  motor 152 

three-phase,  a.c.  generator.. . .   277 

Long-shunt  d.c.,  generator 11 

Losses,   d.c.   motors  and  generators, 

sources 27 

testing  motor 289 

Lug,  testing,  "forked" 294 

with  open  hole 293 


test 


M 


Machine-tool,   rotary  or  drum  type 

controllers,  d.c.  motor 78 

Magnetic  field,  a.c.  induction  motor. .    191 
flux,    action  of,  in  commutating- 

pole    d.c.   generator 21 

Magnetism,  residual,   permits  shunt 

generator  to  build  up 7 

Magnetization,     field,     determining 

direction 63 

graph  determines  generator  vol- 
tage regulation 

test  d.c.  motor  or  generator, 

how   conducted 151 

test  a.c.  generator 276  . 

Magnet-switch        controllers,        d.c. 

motors 85 

Management,  a.c.  generators 178 

d.c.  generators 50 

motors,  a.c 239 

d.c 70 

Mica,  actual  raising  rare  occurrence .  .    122 

high,  identified 122 

in  commutators 121 

Motor,  motors,  alternating  current 
automatic  starters,  connec- 
tions   260 

bearing  troubles 263 

compensating  winding 213 

compensated  induction 214 

compensator  vs.  resistance  for 
starting    squirrel-cage,    a.c. 

motors 245 

control   equipment.    National 
Electrical  Code  installation 

requirements 239 

high- voltage,  when  no- voltage 
release  compensator  starters 

used  for .   248 

induction  and  repulsion,  prin- 
ciples,     construction      and 

characteristics 190 

induction,  see  also  Motors,  a.c. 
polyphase,  induction. 

balking 269 

bearings,  oil  leakage 273 

bearing  troubles 265,  273 

collector-ring  troubles 271 

connected   to   compensator, 

will  not  start 271 

construction 191 

current  taken  at  instant  of 

starting 194 


Motor,  motors,  alternating   current, 
induction   current,  when  run- 
ning without  load 194 

effects    of    unbalanced  vol- 
tages     270 

efficiency  definition 194 

factors    affecting    perform- 
ance    194 

hunting 272 

improper  end  play 272 

inherently   a  constant  speed 

motor 203 

loss  of  efficiency  at  reduced 

speeds 195 

low       maximum       output, 

causes  and  correction 266 

low  torque  while  starting. .  .   2  65 

magnetic  field 191 

maximum  output,  definition  194 
open  circuit  in  field  or  stator, 

effect 268 

polyphase,    primary    speed 

control 255 

speed  control  with  double 

primary 255 

pull-out  torque 200 

regenerative  feature 203 

relation  between  speed,  fre- 
quency number  poles. ...   201 

rotor,  definition 191 

secondary        speed-control, 

speed-torque  graphs 253 

special  types 192 

squirrel-cage,  compared  with 

wound  rotor 197 

rotor  troubles 269 

starting  compensators . . .   243 
three-phase,     delta-s  tar 

method  of  starting. .  .  .   251 
small  started  by  throwing  on 

line 240 

starting     compensator 

troubles 270 

methods . 240 

stator  or  primary  winding ..    191 
relation  between  speed.  Fre- 
quency, number  poles 201 

winding  faults 268 

wound  rotor,  self-contained 

starters 241 

connections  of  starter. ...   241 
operation  on,  of  self-con^ 

tained  starter 242 

starting 242 

torque  graphs. . .....    199 

locating    a  short-c  ircuited 

coil 274 

management,     starting     and 

controlling  devices  for 239 

neutralizing  winding 214 

operation,  effect  of  changes  in 

voltage  and  frequency 196 

performance  guarantees 197 

polyphase,  induction,  cascade, 

speed  control 257 

causes  of  shutdowns 264 

circuit-breaker  protection.  239 
electrical  behavior — same 

as  transformer 193 

method  of  starting  several 

from  one  compensator.   250 
performance    graph    with 
different        frequencies 

and  voltages 259 

speed  control  by  adjust- 
ing frequency 258 


312 


INDEX 


Motor,  motors,  alternating  current, 
polyphase    speed    control,    by 

adjusting  primary  voltage  2o4 
by  changing  number  ofpoles  256 
by  changing  number  of 

secondary  phases..   261 
to  reverse  direction  of  ro- 
tation    262 

troubles 264 

wound-rotor  and  internal- 
starting-r  esistance 
type,  characteristics.  197 

characteristics 198 

speed  control 252 

by  adjusting  resistance  of 

secondary  circuit. . . .   252 
squirrel-cage  induction,  gen- 
eral characteristics 193 

pump,  connecting  float-control 

automatic  starters. 261 

repulsion 215 

-induction 216 

arranged     for     reversing 

service 218 

starting    induction-running, 

characteristic  graphs 224 

secondary  speed  control 253 

short-circuited  armature  coils, 

"inducer"  for  locating 274 

single-phase,  classification 206 

computation  of  horse-power, 
current,  voltage,  power- 
factor  and  efficiency 226 

float  switch  control 261 

induction,     condenser-com- 
pensator    method      of 

starting 211 

develops  no  starting  tor- 
que when  rotor  is  not 

revolving 207 

performance  data 213 

shading-coil     method     of 

starting 212 

split-phase      method     of 

starting 207 

principle 209 

used     only  for  small 

capacity  motors.  .    210 
starting,  construction ....    208 
torque,    starting    current 
speed  regulation,  single- 
phase     phase-splitting- 
starting 210 

Kimble 221 

may  be  operated  from  any 

circuit 228 

measuring  input 279 

repulsion  induction  variable 

speed m .   218 

-starting  -  and  -  induction 

running 219 

series 224 

universal 225 

variable-speed 219 

slip,  speed  regulation 202 

squirrel-cage  induction,  start- 
ing currents  and  starting 
torques  with  different  im- 
pressed voltages  using  com- 
pensator   246 

starting     compensators,     high 

voltage  large  capacity 247 

synchronous 229 

advantages 230 

any  a.c.  generator  will  oper- 
ate as 229 


Motor,  motors,  alternating  current, 
synchronous,  bearing 

troubles 238 

direct-connected  exciter, 

illustration 230 

disadvantages 231 

hunting 237 

improper  armature  connec- 
tions   238 

induction    motor    bring    to 

speed 233 

relation    between    speed, 

frequency,  number  poles  201 
limiting,     starting    current, 
auto-starters,     compensa- 
tors    232 

methods  starting 232 

open  circuit  in  field,  effect. .   237 

over-heating,  causes 235 

polarity 238 

short-circuit    in    armature, 

effect 237 

starting 231 

difficulties 235 

squirrel-cage  starting  wind- 
ing   233 

startsbutfailstodevelopsuf- 

ficient  torque  procedure. .   235 
switchboard  connections.  .  .   232 

temperature  test 278-279 

troubles,  summary  of 234 

uses 231 

testing 276 

three-phase.accurate  input  test, 

"two-wattmeter"  method  281 
coil,   wound  rotor,  starting 

arrangement 243 

computation  of  horse-power, 
current,  voltage,  power- 
factor,  efficiency 204 

determination  of  input  by 
voltmeter  and  ammeter 

method,  test 280 

induction,  temperature  test.   286 
input    test,    method    where 
neutral     motor     is 

brought  out 285 

polyphase-w  attmeter 

method 282 

"one-wattmeter"  method  283 
"  one-wattmeter-  a  n  d-  Y- 

box"  method 284 

troubles 263 

two-phase,      computation     of 
horse-power,     current,     vol- 
tage, power-factor,  efficiency  205 
vertical,     induction,     cement- 
mill,  construction 192 

Wagner  repulsion-starting-and 

-induction-running 222 

wound-rotor,    starting    switch 

troubles 265 

and  its  starter,    National   Elec- 
trical  Code  protection  rules.  .      76 

an  inverted  generator 70 

bearings,  grooves  to  prevent  oil 

leakage 274 

bearing  troubles 143 

Motor,  motors,  direct  current. 

adjustable  speed,  classification     47 

armature,  handling 68 

automatic  starter 83 

brush  glowing  and  pitting, 

remedy 114 

commutating-pole,       overload 

capacity 34 


INDEX 


313 


Motor,  motors,  direct  current. 

commutating   pole,    principal 

advantage  of 16 

shunt,  speed  regulation.  ...      41 
commutation  determines  over- 
load capacity 34 

compound,       connections      of 

regulating  controller 83 

speed  characteristics 44 

control 78 

-wound,  differential  and 
cumulative,  description.  .  45 

construction 33 

control,  armature,  disadvan- 
tages   94 

rheostat 70 

with  flywheel  motor-genera- 
tor   99 

controller,  magnet-switch 85 

crane  controllers,  arrangement     95 
differential    and    cumulative- 
compound  windings 14 

directions  for  starting 72 

stopping 73 

drying  out.... 68 

dynamic  braking 97 

field  flux,  test 142 

frame     castings,     blow     holes 
sometimes  cause  sparking. .    126 

horse-power  output 31 

how    to    reverse    direction    of 

rotation 59 

magnetization-graph  test,  how 

conducted 151 

management  and  starting  and 

controlling  devices 70 

brushes  and  commutator.  . .     66 
motor-generator      with      a.c. 

generators 302 

multipolar,  definition 15 

multi-switch  starter 84 

non-automatic-starting       and 
speed-adjusting  rheostat. ..      81 

performance  data 29,  34 

graphs,  series-wound  method 

of  reading 38 

guarantees 29 

placing  field  coils 65 

poor  connection  between  bar 

and  coil  leads 137 

procedure  when  it  will  not  start 
whenstartingboxisoperated  142 
pressure  -  regulator  -  control, 

automatic  starter,  connec-  102 
ting  regulator,    speed-arma- 
ture control 93 

reliance    adjustable-speed,    il- 
lustration       42 

rheostat  arrangement  of  one 
starting  and  speed  ad- 
justing for  control  of  two 

motors 92 

shunt-  and  compound- 
wound,  starting  and 
speed  adjusting  (field- 
control) 89 

rotary,  drum  or  machine-tool 

type  controllers 78 

runs  in  wrong  direction,  pro- 
cedure     142 

series,  performance  graph 36 

regulating  controller 82 

shunt,      compound,      speed 

characteristics 34 

and    series-wound,    start- 
ing rheostats 47 


Motor,  motors,  direct  current, 

series,  shunted  armature  con- 
nection        77 

series  and  compound-wound, 
speed  characteristics, 

graphic  comparison 46 

shunting  field. , 77 

speed  at  no-load 38 

speed-torque  characteristics.     37 
speed  variation  with  load ...      76 
shunt,     effect  shifting  brushes 

on  speed 41 

illustration,  connections. ...     39 

speed  characteristics 40 

load  and  speed  test 152 

series  and  compound,   con- 
nection diagrams 73 

speed  characteristics 42 

control 77 

temperature    test,     loading 

back  method 152 

speed,  definition 35 

proportional  to  voltage 34 

regulation,    effect    of    com- 
mutating poles 46 

variation,  definition 35 

starting  panels 75 

rheostat 71 

with      multi-switch    starter     85 

sources  of  losses 27 

testing 148 

to  compute  kilowatt  input, 
horse-power,  output,  effi- 
ciency, impressed  voltage  or 

current 48 

torque,    proportional    to    field 

conductors  and  current 34 

,        troubles 105-112 

brush  glowing  and  pitting.  .    114 

Motor-drive  power,  test 288 

Motor-generator,  a.c.  generator  d.c. 

motor 302 

definition  and  construction 301 

flywheel,     equalizer,    control    of 

d.c.  motors 99 

function 101 

frequency  changer,  construction  303 

changes 301 

induction-motor  drive 302 

synchronous-motor  drive 302 

Motor,   horsepower  determined  with 

prpny  brake 149 

required,  often  over-estimated.  288 

torque  and  speed  formulas.  ...    148 

induction,  computation  of  slip.  .   202 

drive,  motor  generator 302 

insulation    resistance,    measure- 
ment     155 

nameplate 148 

output',  to  determine 148 

portable,  arranged  to  determine 

power  required  by  machines. .   290 
power  input  tests,  graphic  instru- 
ments for  recording 295 

required,  importance  of  accur- 
ate determination 288 

of  various  capacities,  inputs,  one 
graphic  instrument  can  be  used 

to  record 297 

synchronous,     drive    for    motor 

generator 302 

testing,  connections 292 

losses 289 

test,  portable,  workable  arrange- 
ments    290 

equipped  with  counter  shaft  291 


314 


INDEX 


Motor,    torque,  function 101 

vertical,  ball  bearings 146 

Multiplolar  generators  d.c.,   number 

of  brush  sets 15 

machine,  definition 15 

Multi-switch    starter,    starting    d.c. 

motor 85 

d.c.  motors 84 

•  N 

Nameplate,  motors 148 

National    Electrical     Code    require- 
ments,    a.c.     motor,     control 

equipment,  installation 239 

Rules,  protection  of  motor  and 

starter 76 

Neutralizing  winding  a.c.  motor.  . . .   214 
Noise  caused  by  armature  unbalance.   146 


Oil    leakage,    a.c.,    induction  motor 

bearings 273 

motor    bearings,    grooves    to 

prevent 274 

rings,  bearing,  to  prevent  stick- 
ing    144 

Open-circuited  field  coil,  locating. . . .    141 
circuits,  flying  in  d.c.  armature 

windings 140 

Operation  of  commutating-pole,  d.o. 

generators 18 

of    non-automatic-starting    and 

speed-adjusting  rheostat 82 

parallel,  a.c.  generators,  division 

of  load 182 

d.c.,  shunt  generators 50 

successful,      a.c.      generators, 

requirements 182 

Output,  a.c.  motors  test  to  determine.  278 
kilovolt-amperes  current  or  vol- 
tage, a.c.,  single-phase 
generator,  computation  174 
three-phase   generator, 

computation 174 

power  factor  or  efficiency, 
three-phase,  a.c.  generator 
horse^power  required,  com- 
putation   175 

kilowatt  a.c.,  single-phase  gen- 
erator, computation 173 

voltage,  current  or  power 
factor,  a.c.  generators,  com- 
putation   176 

low    maximum,    a.c.,    induction 

motors,  causes  and  correction.   266 
of  d.c.  motors,  effect  of  torque 

and  speed 34 

of  motor,  to  determine 148 

Over-compounded  d.c.  generator 9 


Panels,  starting,  d.c.  motors 75 

Parallel    operation,    a.c.    generators, 

division  of  load 182 

hunting  prevents 182 

successful,    a.c.    generators,    re- 
quirements    182 

Performance  data,  compound-wound 
d.c.  commutating-pole  gen- 
erators    28 

d.c.  motors. . . : 34 

single-phase      a.c.      induction 

motor 213 

graph  d.c.  series  motors 36 


Performance   data,    guarantees,    a.c. 

generators 166 

a.o.  motors 197 

d.c.  generators 28 

specifications  for  generators 1 

Phasing  out 1 79 

three-phase  circuits 180 

Pitting  and  glowing  of  carbon  brushes  126 
Planimeter,  energy  ascertained  from 

graphic  power  record 298 

Polarity,  field  coil,  illustration 63 

testing 62 

synchronous  a.c.  motors 238 

tester,  field,  inductive 64 

testing  fcr,  d.c.  generators 54 

Polarities,    commutating-pole    wind- 
ings, determining 66 

Pole,  classification  of  d.c.  generators.      16 

commutating,  object 17 

windings  of 2Q 

number  on  d.c.  generators 15 

relation    between    speed    and 
frequency,  a.c.  synchronous 

induction  motors 201 

speed     and     frequency,     a.c. 
generator,  relation  between, 

formulas 164 

Polyphase    induction     motors,     see 

Motors,  polyphase-induction. 
Polyphase  motors,   see  Motors,  a.c., 

polyphase. 

Power-factor,     computation    single- 
phase  a.c.  motor 226 

two-phase  motor  a.c 205 

efficiency,  horse-power,  current, 
voltage,  computation  of, 

three-phase  motor 204 

voltage  or  current,  a.c.  three- 
phase  generator,  horse-power 

required,  computation 176 

low,  results 230 

synchronous  motor  corrects. ..  230 
or     efficiency,     kilowatt-ampere 
output,  horse-power  required, 
a.c.      three-phase      generator, 

computation 175 

voltage,  current,  efficiency,  a.c. 
generator,  horse-power  re- 
quired to  drive,  computa- 
tion   172 

voltage  or  current,  kilowatt  out- 
put, a.c.  generator,  compu- 
tation   176 

Power,  motor-drive,  test 288 

motor,   input  tests,   graphic  in- 
struments for  recording.  ...   295 
required,  importance  of  accur- 
ate determination 288 

record,    graphic,    energy    ascer- 
tained with  a  plani meter 298 

required,  to  drive  machine  test 

for  procedure 291 

to  drive  a  load,  to  determine 

by  test 289 

Pressure,  carbon  brush 124 

-regulator-control,     d.o.    motor, 

automatic  starter,  connecting.   102 
Primary  or  stator  winding,   a.c.   in- 
duction motor 191 

Principles,    C9nstruction    and    char- 
acteristics, induction  and  repulsion 

a.c.  motors 190 

Prony  brake,  different  forms 149 

motor    horse-power   determined 

with 149 

torque  concept  basis 149 


INDEX 


315 


Pull-out  torque,  a.c.  induction  motor.    200 


Quarter-phase,  see  Two-phase. 


Rack  for  d.c.  armatures 147 

Record,  graphic  power,  energy  ascer- 
tained with  a  planimeter 298 

Regenerative  feature,  a.c.  induction 

motor 203 

Regulating  controller,  see  Controller, 

regulating. 

Regulation,  automatic,  of  d.c.  con- 
stant-current generator 3 

of  d.c.  constant-current  genera- 
tor by  brush  shifting 4 

speed,  a.c.  induction  motors 202 

comtnutating-pole,  d.c.  shunt 

motor 41 

d.c.  motors,  effect  of  commu- 

t  at  ing  poles 46 

definition 35 

how  expressed 35 

voltage,  compound  d.c.  genera- 
tor     magnetization     graph 

determines 13 

d.c.,   generator    operating    at 

different  speeds 12 

definition 1 

Regulator,  armature  control,  con- 
struction   93 

speed,      armature     control,     93 

operation 94 

Reliance  d.c.  adjustable  speed  motor.     42 
Repulsion,      induction      motor,     see 

Motor,  a.c.  repulsion  induction. 
Repulsion   motors,   see   Motors,   a.c., 
repulsion. 

Resistance,    brush  contact 124 

for    starting    squirrel-cage    a.c. 

motors 245 

insulation,  generators  and  motors, 

measurement 155 

complete  machine,  method  of 

increasing 155 

measured  with  high-resistance 

voltmeter 155 

test,  a.c.  generators 277 

Resistors  and  switches,  field  discharge, 
for  automatically  discharging 
field  circuits,  a.c.  generators..  171 

field-discharge 102 

Rheostat  controller 72 

field,      compound-wound      d.c., 

generators 10 

non-automatic       starting       and 

speed-adjusting,  d.c.  motors.     81 
non-automatic-s  t  a  r  t  i  n  g    and 

speed-adjusting,    operation.  .  .     82 
starting     and     speed- adjusting, 
arrangement  of  one  for 
control     of     two     d.c. 

motors 92 

construction 89 

(fi  e  1  d  -control,    for    d:c. 
shunt    and    compound 

motors 89 

operation 92 

starting,  arcing  devices 74 

d.c.  motor 71 

low-voltage  release  device ....     74 

overload  release  device 75 

shunt-,  compound-  and  series- 
wound  d.c.  motors 74 


Ring  fire,  sparking 128 

Rings,  oil,  to  prevent  sticking 144 

Rotary,  drum  or  machine-tool  type 

controllers,  d.c.  motors 78 

Rotor,  a.c.  induction  motor,  definition  191 

or  field,  a.c.,  turbo  generator 1C1 

troubles,  a.c.,  induction,  squirrel- 
cage  motors 269 

S 

Sandpaper,  to  smooth  commutator*.    118 

Segments,  commutator,  loose 115 

Separately  excited  d.c.  generator,  see 

Generators,  d.c.,  separately  excited..       4 
Series  fields,  equalizer  buses  required 

for  three-wire  machines 58 

generators,   see  Generators,   d.c., 

series  wound. 
motor,    see    Motors,    d.c.,   series 

wound. 

shunt  for  compound-wound  gen- 
erators, components  of 15 

on  d.c.  machines,  form  of 15 

-wound  d.c.,  armatures,  number 

of  brush  sets  in 15 

generators,    see    Generators, 

d.c.,  series  wound. 

motors,     see     Motors,    d.c., 

series  wound. 
Shading-coil  method  of  starting  a.c. 

induction  motors 212 

Short-circuited  a.c.   motor  or  genera- 
tor armature  coils,  "inducer" 

for  locating 274 

-circuits,  armature,  test 138 

flying,  in  d.c.  armature  wind- 
ings     140 

-shunt     d.c.,    compound-wound 

generator 11 

Shunt  d.c.,  generators,  parallel  opera- 
tion, connections 51 

dynamo,  see  Generator,  shunt. 
motor,    see    Motors,    d.c.,    shunt 

wound. 

series,  for  compound-wound  gen- 
erators, components  of 15 

on  d.c.  machines,  form  of.  ...      15 
Shunt-wound    generators,  see  Genera- 
tors, d.c.,  shunt  wound. 
Shunted  armature  connection,  series 

d.c.  motor 77 

Shunting  field,  series  d.c.  motor 77 

Shutdowns,    causes,    a.c.,    polyphase 

induction  motors 264 

Single-phase  a.c.  motors,  see  Motors, 

a.c.,  single-phase. 
generator,     see    Generator,    a.c., 

single  phase. 
Six-phase    a.c.    generators,    grouping 

of  coils 167 

Slip,  a.c.  induction  motors 202 

computation  of 202 

Slotted  commutator,  see  Commutator 
slotted. 

Slotting,  commutators 119 

reason 120 

what  it  accomplishes 120 

Sparking,  blow  holes  in  frame  cast- 
ings sometimes  cause 126 

brush,  causes 125 

due  to  open  armature  circuit 128 

rough  commutator 114 

ring  fire 128 

Specifications,  performance,  for  gen- 
erators         1 


316 


INDEX 


Speed-adjusting    and   starting    rheo- 
stats, see  Rheostats,  Speed  ad- 
justing and  starting. 
characterist  cs,     compound- 

wound,  d.c.  motor 44 

d.c.  motors,  series,  shunt,  com- 
pound        34 

shunt  motors 42 

illustration 40 

series  and  compound-wound 
motors,  graphic  compari- 
son   46 

control,   a.c.,    polyphase,    induc- 
tion motor  by  adjusting 

frequency 258 

by      adjusting      primary 

voltage 254 

by     changing    number  of 

secondary  phases 261 

poles 256 

with  double  primary 255 

motors 252 

by  adjusting  resistance  of 

secondary  circuit 252 

cascade  induction  motor  257 

compound  d.c.  motors 78 

primary,  a.c.,  polyphase,  in- 
duction motor 255 

secondary,  a.c.  motor 253 

shunt  d.c.  motors 77 

d.c.    motors  proportional  to  vol- 
tage       34 

variation 35 

series  motor,  no-load 38 

formulas,  motors 148 

frequency  and  number  of  poles, 
a.c.  generator,  relation 
between,  formulas: ....    164 
synchronous     induction 

motor 201 

motor,  definition 35 

normal,  d.c.  generator  operating 

below 14 

regulation,  a.c.  induction  motor, 

slip 202 

commutating-pole,  shunt,  d.c. 

motor 41 

definition 35 

d.c.  motors,  effect  of  commuta- 

ting  poles 46 

how  expressed 35 

single-phase  a.c.  induction 
motor,  phase-splitting-start- 
ing   210 

regulators,  armature  control 93 

operation 94 

test,  d.c.  shunt  nu>tor 152 

-torque   characteristics  of  series 

d.c.  motor 37 

graphs,  secondary  speed-con- 
trol a.c.,  induction  motoT. ..  253 

turbo  alternators 162 

variation,  motor,  definition 35 

with    load    d.c.    series-wound 

motors 76 

Speeds,  different,  voltage  regulation 

of  d.c.  generator 12 

Squirrel-cage   induction    motors,   see 
Motors,          a.c.,         induction, 

squirrel-cage. 

starting    windings,   a.c.  synchro- 
nous motor 233 

Stand,  portable,  graphic  instruments.   297 
Starters,  automatic,  a.c.  motors,  con- 
nections     260 


Starters,  float-control,  a.c.  pump  mo- 
tors connecting 261 

pressure-regulator-control,  d.c. 

motor,  connecting 102 

connections,    wound    motor   a.c. 

induction  rotor 24 1 

motor,  automatic,  principle 83 

multi-switch,  d.c.  motors 84 

starting  d.c.  motor 85 

self-contained,  wound  rotor  a.c. 

induction  motors 241 

Starting,  a.c.,  induction  wound  rotor 

motor 242 

methods 240 

and   speed    adjusting   rheostats, 
see  Rheostats,  starting  and 
speed  adjusting. 

arrangement,  three-phase  coil- 
wound  rotor  a.c.  motor 243 

compensators  for  induction  mo- 
tors, a.c.,  see  Compensators, 
starting. 

current,  a.c.  motors 200 

single-phase,  a.c.  induction 
motor,  phase-splitting-start- 
ing   210 

squirrel-cage  induction  a.o. 
motors,  with  different  im- 
pressed voltages  using  com- 
pensator starter 246 

devices  for  a.c.  motors 239 

panels,  d.c.  motors 75 

rheostat,  see  Rheostat,  starting. 
single-phase  a.c.  induction  motor, 

split-phase  method 207 

small  a.c.   induction  motor,  by 

throwing  on  line 240 

switch  troubles,  a.c.,  wound- 
rotor  motors 265 

synchronous  a.c.  motor 231 

three-phase,  squirrel-cage  a.c. 
induction  motors,  delta-star 

method 251 

torque,  a.c.  motors 200 

torques,  squirrel-cage  induction 
a.c.  motors,  with  different  im- 
pressed voltages  using  com- 
pensator starter 246 

Stator  a.c.,  or  armature  turbo  genera- 
tor, illustration 161 

or  primary  winding,  a.c.  induc- 
tion motor 191 

Steam  engine,  overload  capacity.  ...      33 
turbine,  operates  at  high  speeds.    162 

Surging,  a.c.  generators 186 

Switchboard  connections,  three-wire 

d.c.  generators 58 

Switchboards,  synchronous  a.c.  motor  232 
diagram,  generators  operating  in 

parallel 184 

Switch,   centrifugal,  single-phase  in- 
duction motor 209 

control,   float,    a.c.    single-phase 

motor 261 

float 101 

knife,  for  shunting  out  ammeter.   294 
starting,    troubles,    a.c.    wound- 
rotor  motors 265 

tank 101 

Switches,  field-discharge 102 

and  resistors  for  automatic- 
ally discharging  field  cir- 
cuits, a.c.  generators 171 

field  relay 88 

not  placed  in  circuits  connecting 
collector  rings  to  balance  coils.  29 


INDEX 


317 


Sychronizing,  a.c.  generators  require- 
ments   178 

connections,  more  than  two  three 

phase  generators 180 

dark  or  light,  comparison 181 

definition 178 

lamps 181 

single-phase  circuit  with  lamps, 

principle 178 

with  lamps,  circuits  for 179 

Synchronous     condensers,    see    Con- 
densers a.c.  synchronous 229 

impedance  test,  a.c.  generator.  . .  276 
motors,  see  Motors  a.c.,  synchronous. 

Synchroscopes • 182 


Tank  switch 101 

Telephone  receiver  detector  used  with 

inducer 135 

Temperature,  ambient,  definition 166 

d.c.,  generator  effected  by  speed 

changes 14 

rise,  maximum,  a.c.  generators.  .    166 
test,  a.c.,  three-phase  induction 

motor 286 

d.c.  shunt  motor  or  generator, 

loading  back  method 152 

large  three  phase,  a.c.  genera- 
tor or  synchronous  motor. .   278 
three-phase   a.c.   generator   or 

synchronous  motor 279 

Terminal,  exploring,   and  test  lamp, 

convenient  arrangement 136 

Test,    a.c.,  three-phase  motor,  deter- 
mination of  input  by  voltmeter 

and  ammeter  method .   280 

armature,  "  bar  to  bar  " 138 

short-circuits 138 

excitation  or  magnetization,  a.c. 

generator 276 

external  characteristic  of  shunt 

d.c.  generator 154 

input,  a.c.,  three-phase  motor, 
accurate  "watt- 
meter" method 28 

method      where      neutral 

motor  is  brought  out.  .    285 
"one- watt  meter-and-Y- 

box"  method 284 

"one- wattmeter"  method  283 
polyphase-wattmeter 

method 282 

motor  power,   graphic  instru- 
ments for  recording 295 

insulation  resistance,  a.c.  genera- 
tors    277 

load  and  speed,  d.c.  shunt  motor  152 
three-phase,  a.c.  generator.  .  .    277 

lug,  with  open  hole 293 

magnetization-graph,  d.c.  motor 
or  generator,  how  conducted.  .    151 

motor-drive  power 288 

motors,    portable,    workable   ar- 
rangements    290 

of  power  required  to  drive  ma- 
chine, procedure 291 

open  armature  circuits 137 

polarity,  field 64 

practical  determination  of  torque 

required  to  drive  or  start  a  load  299 
synchronous-impedance,  a.c.gen- 

erator 276 

temperature,     a.c.,     three-phase 

induction  motor. 286 


Test,  temperature,  d.c.  shunt  motor 
or    generator,  loading  back 

method 152 

large  three-phase,  a.c.  genera- 
tor or  synchronous  motor.  .   278 
three-phase  a.c.  generator  or 

synchronous  a.c.  motor.  .  .  .    279 
to  determine  horse-power  output 

and  torque,  a.c.  motors.  .  .  .   278 
power  required  to  drive  a  load .   289 
lamp    and    exploring    terminal, 

convenient  arrangement 136 

Testing  a.c.  generators  and  motors.  .   276 
armatures  where  only  a.c.  or  low- 
voltage  cells  are  available.  .    132 

with  high-voltage  a.c 1 32 

d.c.  generators  and  motors 148 

lug,  "forked" 294 

motor,  connections 292 

losses 289 

Three-phase  generator,  see  Generator, 

a.c.  single-phase. 
Three-wire  generators,  see  Generators, 

d.c.,  three  wire. 

Torque,  a.c.,  motors  test  to  determine  278 
d.c.  motors,  effect  of  field,  con- 
ductors and  current  on 34 

formulas,  motors 148 

low,     starting     a.c.,     induction 

motors 265 

motor,  function 101 

prony  brake  concept,  basis  for.  .    1 19 
pull-put  a.c.  induction  motor.  .  .   200 
required    for    belt    drive,    deter- 
mination with  a  spring  bal- 
ance  N. 299 

to  drive  or  start  a  load,  prac- 
tical test  determination 299 

single-phase  a.c.  induction  motor, 

phase-splitting-starting 210 

speed,  graphs,  secondary  speed- 
control  a.c.,  induction  motor.  .   253 

starting,  a.c.  motors 200 

single-phase  a.c.  induction 
motor  developes  no,  when 
its  rotor  is  not  revolving.  .  .  207 
squirrel-cage  induction  a.c. 
motors,  with  different  im- 
pressed voltages  using  com- 
pensator starter 246 

when    a.c.    synchronous    motor 
starts    but    fails    to    develop 

sufficient,  procedure 235 

Troubles,  a.c.  generators  and  motors .   263 
bearing,     a.c.     generators     and 

motprs 263 

induction  motors 265 

motors  and  generators 1 43 

synchronous  a.c.  motors. .....   238 

field,  a.c.  synchronous  motor.  .  .   235 
starting  switch,  a.c.,  wound-rotor 

motors 265 

synchronous    a.c.    motor,    sum- 
mary     234 

Truing  commutator  with  file 118 

Turbo    generator,  see  Generator,  a.c. 

turbo. 

Two-phase  generator,  see  Generator, 
a.c.  sinyle-phase. 

U 
Universal  motor,  a.c.,  single-phase. .  225 

V 
Ventilation,  turbo  alternators 162 


318 


INDEX 


Vertical  waterwheel  generator,  a.c., 

construction 163 

Voltage,  a.c.  motor  operation,  effect 

of  changes 196 

computation     single. phase    a.c. 

motor 226 

two-phase  motor  a.c 205 

current,  efficiency  or  power 
factor,  a.c.,  three-phase 
generator,  horse-power 
required,  computation, 
generator,  horsepower  re- 
quired to  drive,  com- 
putation  

power  factor,  kilowatt  output, 

a.c.  generator,  computation.    176 
drop,  due  to  contact  resistance, 
increased  if  fit  of  brushes  or 

commutator  is  poor 

impressed,  d.c.   motor,  to  com- 
pute   

or  current,  a.c.,  single-phase  gen- 
erator, kilovolt-amperes  out- 
put, computation 174 

three-phase  generator,  kilo- 
volt-amperes  output,  com- 
putation   174 

power-factor,   efficiency     horse- 
power current,  computation  of, 

three-phase  motor 204 

regulation,  definition 1 

d.c.    generator    operating    at 

different  speeds 12 

shunt-wound  d.c.,  generators. . .       8 
terminal,  increase  due  to  over- 
compounding 13 


176 


172 


125 

48 


Voltages  and  frequencies,  different, 
a.c.,  polyphase  induction  mo- 
tor performance  graph 259 

Voltages  unbalanced,  a.c.,  induction 

motors,  effects 270 

Voltmeter,  high-resistance,  insula- 
tion resistance  measured  with. ...  155 

W 

Wagner       repulsion-starting-and-in- 

duction-running  a.c.  motor 222 

Water-cooled     machinery,     standard 

temperature  ratings 30 

Waterwheel  generator,  vertical  a.c., 

construction 163 

Wattmeters,  graphic  ammeters  used 

instead 296 

Winding,  commutating  poles 20 

faults,  a.c.,  induction  motors 268 

stator   or   primary,  a.c.    induction 

motor 191 

Windings,  armature,  d.c.,  flying 
grounds,  short-circuits  and 

open  circuits 140 

commutating-pole,     determining 

polarities 66 

cumulative-and-differential-com- 

pound,  for  d.c.  machines 14 

danger  of  overheating  when  dry- 
ing       68 

differential-and-cumulative-com- 

pound,  for  d.c.  machines 14 

field,  on  d.c.   generator  frames, 

direction  of,  illustration 15 

Wound-rotor  a.c.  motors,  see  Motors, 
a.c.,  wound-rotor. 


SS-SSw  sl-°° 

OVERDUE. 


VC  40384 


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